1. Northern Exposure - I loved this show when it was originally on, and it's one of my favorite TV shows ever. It's fun, it's funny, it's got great characters, it's got great plots, I really can't praise it enough. It's one of those shows that makes you feel good after watching it more often than not.
2. Alias - If you like the pacing and twists in Prison Break then you'll love Alias. The first two seasons were the best, and I admit it took a downhill turn in season 3 when they started focusing too much on the romance (IMO), but it was still decent. The last season sucked, but it at least wrapped things up.
3. Desperate Housewives - I like this show because it has a really dark, sharp sense of humor. It's not to everyone's taste, but it's fun and a guilty pleasure. Andrew VanDeKamp is a great semi-villain, and they've had some good actors, like Kyle MacLachlin.
4. Dead Like Me - another smart, funny show. I really loved its take on office life, and the idea of grim reapers as drones just doing their jobs. The characters are fleshed out well, and they're all interesting. I can't believe Showtime canceled it.
5. 24 - It's got some fast pacing, like Prison Break, and some unbelievable stuff, again like Prison Break, but it moves so quickly that it's easy to forgive. There are plenty of plot twists as well, if you like that kind of thing. The early seasons were the best, I kind of lost interest around Season 4. Still, it's mindless fun.
Also, it's not coming out on DVD until August, but if you haven't been watching Heroes than I can't recommend it enough. One of the best shows out there, really.