Seasons 1 and 2 are unarguably the best. Season 3 had some fun but was obviously weaker than the first two. Season 4 touched pretty much every feeling, although I got the feeling they were trying to juggle more things than they could they still managed to land all the pieces without breaking anything. And I thought Season 5 was a return to form for both Dexter and us, and I'm excited for Season 6.
Oops sorry I hit send too quickly. I meant to touch on the books next. The show left the books behind after the first two novels, and reading the third one kind of shows you why. I don't want to spoil it or anything, but the plot is quite... out there. Personality-wise very little is different except how certain situations effect him. It's hard to be vague and specific at the same time... But if you want more Dexter after you watch all the shows, the books would be the best place to look. Did that sound as stupid to you as it did to me? I think I need to sleep.