I thought I’d try to divide my fandoms by archtypes 
Puzzles
I always loved being challenged by what I didn’t know. Started young with riddles/puns, read with delight the Time Travel choose-your-own-adventure books, and moved quickly into Infocom games. Was amazed with Myst and Riven (with associated novels and comic book), and thoroughly enjoyed Journeyman Project 3.
Geek Humor
Things that have geek in-jokes, have actual technology jokes in them, and arcania humor appeals to me, since I love obscure knowledge. The Hitchhiker’s Guide to the Galaxy was a defining moment in my life and I went for a while where I would get grumpy if I hadn’t read/listened to/watched/played one of the version within a six month time period. MST3K follows in that theme. I watch Futurama virtually every night and never seem to get tired of it (top two episodes have to do with holophoners ;)). FoxTrot is like the only major comic that I know of that features executible code that makes me laugh. And Animaniacs… let’s just say that I know too many of those songs by heart for my wife’s comfort and she seriously hopes they never come out on DVD.
Mysteries
It’s part of the puzzle thing, really, but I love mysteries. Many people think that since I am a geek, that I’m obviously obsessive about scifi or fantasy. Wrong. I may have started with the Hardy boys, but I quickly found that The Three Investigators and Tintin where more my style. I’ve read nearly every Nero Wolfe more than three times (and found the Timothy Hutton TV series to be excellent). I miss big screen mysteries that aren’t about something besides the mystery (dunno if that makes sense)… the only good one that I can think about is The Zero Effect, which I obsessed over for quite some time. Currently, I get most of my mystery fulfillment from TV: CSI, Monk, and Detective Conan (aka “Case Closed”). Oddly, there were never any video game mysteries that really appealed to me, until I found Tex Murphy. It also kind of had story arc elements, but they’ll probably never be completed.
Story Arcs
I seem to have a weak-spot for books/TV shows/movie that stretch out a storyline. If I get started on something, I have the tendency to want to know everything about it and see it through to its conclusion, especially when the author has everything plotted out beforehand, has crafted a complex backstory, drops little subtle hints from time to time (which means I’m a Star Wars geek as well). This has made me a huge fan of the Dark Tower series and the first 2/3 of The Wheel of Time series (the fact that I keep reading after that shows just how weak my spot is). I’m kind of getting hooked on Song of Fire and Ice. Along with that go Harry Potter and Lemony Snicket. TV shows include The Pretender, with a great mix of humor, sexiness, and “You can be anything you want to be” themes; Alias, with sexiness, plots within plots, and the whole “What was that Italian guy really up to?”; 24, which, in addition to the story arc, seems to have a lot of fun with breaking into new areas of TV viewership. The only story arc video games that have attracted me are Final Fantasy VII/X, and Jak and Daxter. Story arcs are also one of the reasons I got hooked on anime: Big O, Inuyasha, Witchhunter Robin, and FLCL. Fullmetal Alchemist is also beginning to exert its pull over me. 
The only other fandom that I have that doesn’t fall into any of these categories is Lupin III. I think the fact that it was the first cartoon that I ever saw with adult themes (although the main character is sometimes really pretty juvinille) might have something to do with that. It’s sense of humor and character interaction also play a big role in my enjoyment of it, I guess.