With regards to a TV series it would be without a doubt Oz. Brilliant, just brilliant. 
You’re not the only one. I can’t say that I am its biggest fan, but once I started watching it, I didn’t want to stop. I discovered the show early this year and “mainlined” Roswell episodes, watching them back-to-back, until I finished the entire series in a frighteningly small amount of time.
So yeah, I guess that means it was addictive.
I tend to like flawed but intersting characters, “anti-heroes” if you will, so I dig Firefly, Farscape, DS9, etc. Star Trek (both TOS and TNG) rarely delved as deeply into the dark side of human nature, and neither Voyager or Enterprise have caught my attention.
Futurama rocks for it’s sheer zaniness (I’d love to see Bender kick the ever-loving crap out of C-3PO!), something the Simpsons have kind of lost. Red Dwarf falls into this category, too.
For books, David Drake, S.M. Stirling, David Weber, Keith Laumer, Glen Cook, Eric Flint, Larry Niven, Patrick O’Brian, and John Ringo make up most of my paperback fiction collection.
Mr. Drake and Ringo for their vivd, un-sugar-coated depiction of combat, everyone else for just being good storytellers. I like David Weber for one particular larger-than-life heroine, and the plucky star-kingdom she fights for; plus, even his walk-on characters come across as real, complex people. I like Patrick O’Brian for two unlikely friends and their adventures, set against the backdrop of Napoleanic Europe.
Currently, in no particular order: Pixar movies, Transformers, Apple Computers, Terry Pratchett. And San Francisco Rush 2049. Not sure if I can out-Transformer-geek E-Sabbath or not, though Tengu still gets props from me for being the most unflappably cheerful guy to ever grace a BotCon. 
Previously, with occasional indulgances in: The Paranoia role-playing game, the Toon role-playing game, the Nuclear War card game (triple yield on a 100 megaton warhead!), Williams pinball machines, Atari arcade games.
Why? Why?? Because I’m a geek, dammit!
T.V.:
Columbo
Frontline
Star Trek TNG (and TNG only - the rest I just like, TNG I’m a fan)
MST3K
The Daily Show (I’m becoming a fan anyway)
Comics
X-Men
Authors
Louis Menand (lately)
Philip K. Dick (from college years)
Kurt Vonnegut (from teen years)
Susan Cooper (from when I was a child - wrote children’s fantasy)
Music
Elvis Costello
Elliott Smith (becoming a fan)
Ween (sorta fan)
Other
NPR - especially This American Life and Car Talk
Pierce Brosnan (I think it’s just the looks)
Babylon 5 - the best space opera ever filmed, and the most complete TV series ever made.
Anything by Joss Whedon - I have a feeling he made TV shows targeted directly for me.
Billy Joel - yeah, he’s uncool. That’s part of the appeal. Besides, some of my earliest memories are of living in the Village in 1977 and listening to my parents playing the Stranger on their stereo.
Guy Gavriel Kay - I’ve stated before - recently - how much I love his books. One of the best pure writers in the modern fantasy field, and inventor of a whole new genre of “fictionalized history”.
Roger Zelazny - I read the enire first Ambr series before I was 11 years old; his short stories are the best in the SF genre, bar none.
George R.R. Martin - Not only am I looking forward to his next Ice and Fire book with nearly as much anticipation as I have for the approaching birth of my firstborn, I also love his early stuff, including Armageddon Rag and *Tuff Voyaging’**.
Terry Pratchett - Probably the funniest man alive. But even if you’d take out all the humor from his novels, you’d still be left with a damn good writer; in fact, I think the funny stuff is incidental to the real quality of his books.
The Coen Brothers - Even when they’re bad, they’re pretty good. And The Big Lebowski is the funniest film in three decades.
Military history in general
D&D - I’ve ben playing since 1984; I’ve been playing with the same group of people since 1988. We’ve reached the point that our “talk about D&D”/“actually play D&D” ratio is roughly 5:1.
Heh. Between that picture and this post, I think you just became one of my favorite posters.
I’m a huge fan of LotR , bordering on being an obsessive squee-ing fangirl . It’s just not seemly at my age and my family frankly think I’m nuts; but because they are very nice they choose to tolerate it. Although this madness has, I admit, been mostly kindled by the films, I’ve had more than a passing interest in Tolkien since we read The Hobbit as a class in school at the age of 9 or so and played what would now be regarded as a pretty primitive LotR role-playing/board game at University 20 years ago( having battled through the book just once). By contrast I’ve read the book already five times this year. How times and perceptions change!
My continuing interest is rock/indie music of all kinds, from Led Zep and Cream through The Smiths to Muse , the White Stripes and The Kings of Leon . I’ve always been unable to resist a great guitar riff and it will always be so I think. I was also a fan of John Peel who was a constant source of inspiration from my extreme youth into my approaching middle age, as a man as well as a DJ, and his recent death was a great loss to new music; not only in the UK.
Other than that I have a passion for history in general and British Iron Age and Anglo Saxon (pre)history in particular. I’m lucky enough to live within striking distance of some of the best megalithic sites in Europe (Stonehenge, Avebury and the Wiltshire barrows to name but a few) and find them all mesmerising because they are so mysterious.
I could ramble on and on, but I won’t!
I’ll second these obsessions, except that I didn’t really find out about Dick until the last year of college, so it’s more of a grad school obsession. I’ve been just tearing through his books, though–I’ve read fourteen of them in the last year and a half, and the only reason I haven’t read them all by now is because I want to space them out. I can totally recall everything from the books of his that I’ve read, and if you’ve shown even the slightest interest in Dick (even if you’ve just seen Blade Runner that one time)–watch out, I’ll talk your ears off. I’ve never read an author like Dick; by most comparisons he’s a pretty poor writer, but I’m addicted to his style, not to mention the ideas. It doesn’t hurt that his style is similar to mine, or that I have a lot of the same philosophies. He’s totally my writing idol. Anyway, this probably seems way too obsessive so I’ll stop now. But yeah, Philip K. Dick is a major fandom of mine.
I’m not too fixated on a certain TV show… well, I have seen most episodes of Aqua Teen Hunger Force at least five times, and usually have an ATHF-related comeback for every situation (though I don’t always use it). It’s the same with Futurama and The Simpsons, but who isn’t obsessed with those shows?
I’m also way into sushi, and have visited all of the area sushi places several times, trying to order something different each time. The justification for that is just that it’s a food I like to eat.
For Books: Middle Earth. I’ve been reading Tolkien for years and years and I’m never tired of it. I also like Tolkien’s other short stories, poems and I love his art. I have a calendar of his artwork and was squealing with delight when I found it. I had a crush on Legolas long before I ever saw Orlando Bloom. My ultimate favorite is the Tale of Beren and Luthien. I’ve read the Lay of Leithian (as much as has been written) and get all choked up each time I read it in the Silmarillion.
Music: The Beatles. When I was in Jr. High I was looking for the import vinyl LPs so I could have all the British versions of their albums. Pity I was born a year after they broke up. I am compelled by some mysterious force to always stop on a Beatles song when it’s on the radio and I’m scanning. Other bands come and go, but the Beatles will always be.
TV: Dr. Who. Oh I saw all the episodes at least once (and the Tom Baker era too many times to count), and I bought nearly all the books, but when the local PBS affiliate stopped showing it, I stopped watching and I couldn’t keep up with the Who-verse. Still, I was…obsessed for a long time with it.
Computer RPG Games: Might and Magic. I have M&M (VI - X) and Hereos of M&M, and all the Tarnum senarios. I love the M&M world. Enroth, Erathia, Roland and Archibald. I was distressed when they destroyed it, and pulled some of the Heroes into the new dimension, and am even more distressed to hear that 3DO is in bankruptcy and I will probably never get a Heroes V, OR a M&M XI. I know most everyone else loves D&D – which I’ve never been able to complete because I lose interest in the quests – but I play (and replay) Might and Magic.
Jeez, '93? That makes me feel ancient. I started watching TOS in reruns probably 20 years before that. I was such a huge fan I tried to go my entire seventh grade year without showing emotion since I was obviously meant to be Vulcan. I don’t think I cracked a smile once. (I was a strange child.)
Other fandoms (I’m defining fandoms as communities I occasionaly participate in or check out online, not just likes):
Stargate: SG1 and Stargate: Atlantis
Gundam Wing
Lord of the Rings
Radiohead
Sherlock Holmes
By far, my biggest fandom is “yaoi”, an obscure genre of Japanese manga.
I thought I was too old to be a geeky fanboy, but then I met Firefly. I got caught up in the whirlwind of first trying to save the show, then get another network to pick it up, and finally to get the DVDs released. I flushed with pride when a cast member addressed me by name (and I dropped his name here, much to my later shame) and gushed like a schoolgirl when Joss or some other ME–I mean, Mutant Enemy (but SMG will always mean “submachine gun” to me)–bigwig posted on our board. You’d think I was 13 again.
But my obsessions only last so long and when the movie got announced I was happy but felt my work there was done. I rode off into the sunset, trying to act cool but secretly embarassed at how nerdy I had been.
Hmm… lessee… fandoms I’ve geeked out for…
TOS/early TNG Trekker – hey, it’s what fit my time-frame, what can I say? And at least it started off genuine.
Sequential art/Animation/Illustration – Because doodling is my own form of relaxation and expression, I gravitate to that artform and its related idioms. Within it, of course, there are subcategories I pay more attention to:
QUINO (Joaquín Salvador Lavado) – Argentine cartoonist. Mordant observations of the human condition; plus of course, Mafalda: “Peanuts”(from when it was good) meets “Doonesbury”(from when it was good) from 1964-1973.
Anime/Manga – Grew up with the Anime (sometimes we’d get it here before in the States), from Astro Boy on, through Speed Racer, S-5, the White Lion, Marineboy, Harlock… and from the start I noticed these cartoons were… different. Later, I really caught on to the various characteristic styles and graphic designs (e.g. Tezuka, Matsumoto, Takahashi, Shirow, Miyazaki, Oshii, Anno) and the storytelling and never let go.
Beatles – although broken up by the time I was 9, I could tell this would hold up well. By my teens I had become better acquainted with their evolution and also was on the lookput for the EMI-Parlophone LPs with the extra material.
The Who – in this case the original lineup survived 8 more years, so they had more time to catch me up in their rougher, tougher, LOUDER but still high-quality stuff while at the same time containing great lyrics courtesy of Peter. Yeah, I know, as a nerd I should have been into Zep for my loud rock, but hey… why should I care, why should I care?
The Whedonverses - BtVS, Angel, Firefly. Odd, since I didn’t watch any of these shows in their original incarnations, except for *Angel * season five. My fandom is purely via DVD. I may take off work to see the *Serenity * movie on its day of release.
Terry Pratchett - I’ve read most of the books multiple times, can quote passages from memory, have gotten friends hooked on him as well.
I’m a closeted Diana Gabaldon fan. I will only talk about that if I come across another fan, because I’m embarassed that I’m so into a “romance” series.
I never miss an episode of Desperate Housewives and I plan my Tuesdays around The Amazing Race.
I forgot about MST3K. Mike, Joel, Dr. Forrester, Pearl…it doesn’t matter. I love them all.
My primary fandom these days is comics. I’m pretty savvy in that field, and I know a lot about it.
I’m also a massive Survivor fan. I always love the challenges where they test the players on Survivor, because I can nearly always run away with it. Before S4 premiered, I used to be able to name all the contestants in their boot order, but I’m nearly certain I couldn’t do that today without Googling.
I love the Whedonverse, but I’m less a fan as time goes on. I think I’m allowing my feelings about S6 & S7 to corrupt the whole deal, which I know isn’t fair, but there you have it.
In addition to the Whedon shows (Buffy, Angel and Firefly), I’m also a big fan of L. Frank Baum’s original Oz books (but none of the movies or later, non-Baum stories); I enjoy William Goldman’s The Princess Bride (the movie is alright, but the book is sooo much better) and have yet to get tired of re-reading it; I feel the best bad movie out there is The Pirate Movie; and, on a more scholarly level, I enjoy the many thousands of variations of the Faust legend.
I am a die-hard Star Wars fan primarily. It’s the one part of my youth I have never been able to shake. Now I just accept it. I even got Boba Fett’s madalorian skull thingee on my arm as a tattoo. My most recent fan obession is Cowboy Bebop. Never been into anime before but I just started watching the show about 2 years ago and now I have the complete series and the movie on dvd. I am also an Irvine Welsh disciple. I like to liken my own experiences in life to either his books or good old HST.
Video game music, specifically, Nintendo music. Sure, many love Nintendo games, but do you know who composed the music for them? Yeah, I love the RPG soundtracks that everyone loves like Chrono Trigger and Final Fantasy, but also soundtracks for Mario Kart and Kirby and others that everyone ignores. I know, I’m weird.
Other than that I’ve got the basic fandoms over Star Wars, Simpsons, Futurama, Lord of the Rings, and maybe some others.
Academically, I’ve always been an astronomy nut, reading through A Brief History of Time at age 14 and kind of understanding some of it.
Oh, I actually meant the HBO series called Oz that was set in the Oswald correctional facility :smack:
Or was I the only one that liked it? 