CSI
Not sure if 24 counts for me. I missed season one entirely then saw it on dvd before season 2 started.
Gilmore Girls
I guess I don’t watch TV shows consistently.
Star Trek (TOS)
The Prisoner
Survivor (all of them)
The Shield (the best show on TV right now)
Are You Being Served?
Twin Peaks
American Dreams
Gomer Pyle, USMC
Gilligan’s Island
Star Trek
Hill Street Blues
Six Feet Under
The West Wing
Cop Rock (groan!)
Gilligan’s Island
Andy Griffith
The Flintstones
The Brady Bunch
Star Trek
Star Trek: The Next Generation
ST: DS9
Babylon 5
VR5
Buck Roger
Battlestar Gallactica
Highlander
Andromeda
X-Files
Law & Order (the original)
Cheers
Barney Miller (and the spinoff Fish)
MAS*H
Brideshead Revisted
Outer Limits
Twilight Zone
Night Gallery
Dark Shadows (the old and the remake)
Banana Splits
HR PuffnStuff
The Monkees
Fawlty Towers
Blackadder
Stuff I can thank my kids for getting me into:
RoboTech
Smurfs
The Gummi Bears
Stuff I blamed on my kids:
The Real Ghostbusters
Mel Blanc’s Loony Tunes (only been kind half hearted since he died)
The Muppet Show
Ducktales
Cowboy Bebop
Kids in the Hall
This is easier in the UK, where a ‘series’ is often only 6 episodes (or even fewer). For example, I’ve seen every Fawlty Towers, but then again most everyone has, and there were only 12 made.
However, for shows that have sufficient episodes to make the claim worthwhile, my only one would be Mission Impossible. When I was very much younger, I thought this was the best show ever, and I saw every single episode.
Off the top of my head:
Police Squad! (someone has to put this series out on DVD)
Penn & Teller’s Bullshit! (someone has to put this show into prime time)
Transformers (though I admit there are episodes I wish to forget)
The Weird Al Show
Mighty Orbots (definitely needs a DVD collection)
The Twilight Zone
Monty Python’s Flying Circus
All In the Family
GIlligan’s Island
West Wing is the only one I can think of.
Except, I haven’t missed an episode of SNL since 1992 or so. The ones I’m not home for, I either tape, or catch in reruns. But I can’t claim to have seen the show’s entire run.
Fawlty Towers
Monty Python
The Rockford Files
Cracker (the British one)
Homicide
Nero Wolfe (the new cancelled one)
I wish I could say MST3K but sadly I’ve missed a bunch of them.
Hmmm – I forgot Blackadder and The Prisoner. Also (if it counts) the James Burke series – Connections and The Day the Universe Changed, but I’m not sure I caught every episode of the later Connections series.
Seinfeld is the only one I have gone down a list and checked EVERY episode.
Friends was also on my list until this season. I’ve missed a couple of episodes. Same with the Simpsons.
I can’t wait until I can catch up on a few Scrubs episodes I’ve missed.
Now that they are on Adult Swim, I’ll add Family Guy and Futurama to my list.
Seinfeld (probably)
In Living Color
Salute Your Shorts 
24
Roseanne (except maybe the last season, but no one want to count that anyway)
Just two:
Simpsons
My So-Called Life
Twiddle
Crikey! I can’t believe I forgot:
Alias
Dark Angel
Police Squad!
Twin Peaks
–Cliffy
Seinfeld
ST:TOS
ST:TNG
ST:DS9
ST:VOY – a struggle, to be sure.
ST:ENT
Simpsons
Living Color
Futurama
Family Guy
Trading Spaces – do you have any idea how many of these there are?
While You Were Out
Faking It – copout. Only 4 made as a pilot for TLC.
What Not To Wear
Crossing Jordan – my soap opera
Third Watch – my brother’s soap opera
Law & Order
Law & Order: Crime and Punishment
Roseanne
All-American Girl
The Cosby Show
Different World
Ellen
Will & Grace
Probably several more I don’t remember.
God, I’m a loser. With a lot of TiVo Season Passes.
Babylon 5
Kung Fu: The Legend Continues
Star Trek: The Next Generation
Those are the only ones I’m sure of.
Buffy The Vampire Slayer
Today’s Special (Children’s show - I used to cry if I missed it)