I loved season 2, season 1 introduced Dexter so it will be hard for me to see something as better than 1 because I love the show so much, but the inner monologue, the prodding and questioning of who/what he is was phenomenal. I particularly loved the scene in which he has Doakes in his cage and he commits a murder in Doakes presence, then after he is finished Doakes is completely disgusted of just what Dexter is having seen the grim reality. Then Dexter flashes back to when his father walks in on one of his murders, and he realizes that he's not what his father had in mind. He's not an assassin of evil, but a true serial killer who enjoys not the exacting of justice, but the dismembering of his victims. Then after Doakes is dead and his burden of paranoia is lifted as the "Bay Harbor Butcher" is "caught", he laughs at his own weakness, the idea that he might give in, and begins to reconvince himself what he is doing is right.
Season 1 though was great, because it's what made you fall in love with his character, his self-assuredness, his charm, his bravado, his meticulous nature, self-awareness. It was all so brilliantly portrayed by Hall, that you couldn't help, but be captivated by him.