I admit, every season since has been copying season two, which has lead to the show's decline, but...season two is by far my favorite. Season one has that awful lag in the back half filled with Teri's amnesia and other filler on the way to a memorable finale. I like the back half of season two a lot more.
I'd rank Season2 as my favorite followed by Season 1. Seasons 3, 4, 5 and 7 follow in no paticular order, possibly all tied, and season 6 stands at the bottom. So far, though early to tell, Season 8 is looking promising.