If you haven't seen Friday Night Lights (the tv show, not the movie) - you should. It's hands down my favourite show currently on telly.
I was a huge fan of The West Wing and it's probably my favourite show ever. Firefly is right up there, too.
Other current shows I love are Chuck, Castle, Doctor Who, Torchwood & Gossip Girl; classics include Buffy, Angel, Crossing Jordan, and the X-Files.
Really it depends on what you like! The next thing I'm planning to mainline when I get a chance is How I Met Your Mother, which my friends all rave about.