Fringe is very underrated, an absolutely magnificent show but I rarely see it mentioned in such topics. It's a bit unusual because it has a very complex and intriguing overarching storyline, and most episodes focus on that, but they also have some "monster of the week" type episodes. I can only recommend it. It's like X Files but fresh and, in my opinion, with a much more compelling and connected big storyline.
I'm also a big LOST fan. I know a lot of people hated the ending, but I loved it. This show seems to be hit or miss for a lot of people.
Dexter is really good as well, but got bad towards the end.
Scrubs was the best doctor show, in my opinion, and despite the comedy, by far the most realistic one. House MD was pretty cool but started annoying me, and was really unrealistic.
Prison Break is AMAZING in seasons 1 and 2, drops significantly in 3, and got alright again in 4. There's gonna be a new season soon, so I'm very excited about that.
The Walking Dead has an interesting range, from brilliant to very meh. I'm pretty unhappy with some of the pacing, the character deaths, and so on.
In crime shows, my favorite by far is Criminal Minds, followed by Law and Order: SVU.