Season 2 is my favorite. So many great episodes and the tone was really great throughout out that year. Season 4 comes close though also, the brothers had issues and angst but their were alot of powerful episodes and season 5 even if I hated the ending.

Season 6 is a least favorite. I do think season 6 gets a bad rap, since overall it was decent but I think out of all of these I would choose season 6.