Season three. Season one took me forever to get through, and I generally hated it. And I felt season two could have moved along a little quicker. They did change some things from the comics, so why couldn't they have cut out the useless crap for season two? But season three was awesome. Minus the two major character deaths.
Season 1 is the best. The CDC episode alone ***** on seasons 2 and 3 combined. Season 3 was really bad. I'll still be watching, but omg please let them get this show back on track.