I think the first season was so good was because of the ice truck killer. It had you guessing who it was, and when you knew, but Dexter didn't, it kept you watching, wanting Dexter to find out, and it built up to an excellent finale, whereas, season two is more about the aftermath and the discovery of Dex's victims, and seeing how he's going to get out of it, plus, it concentrates more on him trying to answer questions about who he is.
As for fave actor, they're all great. I really like Rita, as she's building her character, becoming stronger, and gives Dexter focus, but I thought about it, and I think I'd have to say Doakes! I'd watch it, and see him following Dexter, and be all, just leave him alone to kill, will ya?! But as season two finished, I realised that he actually won me over, but I won't spoil it for anyone!!...
I won't kill my sister...I won't kill my sister...I won't kill my sister...