And now, onto the show...
FOLLOWING CONTAINS SPOILERS
Season 1 was awesome. I don't know if it has been topped.. The fact that Rudy was Dexter's brother blew my mind. I did have a suspicion that he was the ice truck killer all along though.
Season 2 wasn't as good, but still had some very suspenseful moments. So glad Lila died, stupid cow. I hated how Doakes continually harassed Dexter so I didn't mind seeing him die either - but I didn't mind him towards the end.
Season 3 - Miguel was such a legend.. I loved the character, and was so cut when he died. Him and Dexter could've had something good but of course Miguel couldn't control himself. He should've just stopped after he killed Ellen Wolfe - he was lucky Dexter gave him a chance but he blew it.
Season 4 - to be honest, wasn't that exciting UNTIL episode 9... What a freakin' awesome episode that was. Quite possibly a top 3 episode of the entire series. Episode 10 was also pretty awesome.. (I haven't watched episode 11 or 12 yet, I'm watching Episode 11 tomorrow night and episode 12 next Thursday night. Can't wait)
I couldn't say which has been my favourite season. I would PROBABLY rank it like this:
Season 1 > Season 3 > Season 4 > Season 2
(Season 4 wasn't that great until episode 9. Then shit got real intense, pushing it ahead of Season 2)