Season 1 was the best so far, season 2 was pretty good - probably the second best.
I liked Season 3 but a lot of people didn't care for it. The only problem I had was Jimmy Smit's constant "over-hispanicization" of his character. You'll see what I mean when you watch it.
There are a lot of subtle things going on in Season 3 that make sense by the end, especially once Smit's brother reveals a few things to Dexter. I have a degree in psychology and I love all things psychology and so I loved watching Dexter try to make friends in Season 3 - a lot of people just wanted more vengeance killing.
I personally found Season 4 good but also too long. Still, I wouldn't say it was bad or anything but just a bit different. And the end is quite stunning.
I haven't watched Season 5 so far (been too busy) but I have a few friends who say it has been decent.

Oh, my recommendation is The United states of Tara. It is about a woman who has multiple personalities (it a comedy as well as drama) and follows her and her family as she tries to figure out what trauma caused her personalities. Season one was pretty good by itself but I was pleased when I watched Season 2 and it didn't fall off. Check it out if you get a chance.