Season 4, definitely. Just brilliant. Left me agog. Hehehe, and it's ace because it lets me say 'agog'.
As for fave character, that one's really tough. I can't choose, because I loved many of them for different reasons. So Bubs for his journey, Stringer for his strange way of not being like your typical gangster, Bunk for being Bunk, Omar because he was legendary, Bodie because I, too, loved him by the end, Carver for his growth and the Randy storyline, Norman because his shiiiiiiiiiiiiiiiiiiiiiiit just about beat Clay's in length and awesomeness, Clay Davis because he was one smarmy cheeky motherfucker, Michael because I really felt for him ... I could go on. Oh, and Frank ... I loved Frank so much.