From what I've seen:
3
4
2
1
Big list, yeah. The only difficult part is choosing between Seasons 3 and 4. I *love* 4, because it has Hush, Restless (2 of my favorite eps), Superstar (!), and Primeval. However, Season 3 is more rounded out. Like, I'll probably never watch "Living Conditions", "A New Man", or some other season 4 filler again, but I could watch all of Season 3 without skipping anything.