Season 2 is by far the best in my opinion. How can you not love Ava? She was so evil and yet she still made you love her. Well maybe not evil, but she was a little messed up in the head. I also liked all the directions the characters were thrown into.
Season 1 comes in a close second. It was new and it had nothing to lose. By far the most gutsiest season to date. And Escobar was awesome.
Season 3 a distant third. While there were some I liked, there were some I didn't. The Carver series was finished but it really started in S2, and that's where most of the intrigue for this story line came from. The finale kinda ruined it for me though. Quentin should have been more ruthless, and not so stereotypical as the villian. Kinda reminded me of superhero comics, where all the villians monologue about their plans.
Then Season 4. I had no problem with it. I thought James was an awesome villian, especially after what she did to Reefer(I think that was his name). But honestly do I really need to say what ruined it. Connor 2026 and the departure of Joely Richardson. The least they could have done was found some way for her to go out with a bang.