Friends, Lost, Battlestar Galactica (the newer one), Arrested Development, Firefly, Sports Night, Wonderland (ok, probably doesn't count, as this was cancelled after 3 episodes!), Moral Orel, Family Guy, Cowboy Bebop, Neon Genesis Evangelion....I think that's it, but I'm probably missing shows from when I was younger, I don't know. I realized that the series' I've watched in their entirety, I am also more likely to have watched more than once. They're that good!
EDIT: Had to add Buffy! I watched every single episode for the first time last summer, and was blown away by it! Oh! Also forgot an awesome show I've watched repeatedly, though it's not fiction--Penn & Teller's Bullshit!