That's a really hard question. All of them! I've been actually trying to choose my favorite season for some time now, but I can never choose. But I think if I really had to choose I would say either season 1, 2, 3, 4, 5, or 6.
Seeing as Jensen Ackles and Jared Padalecki are in every season, I'm gonna have to say all of them lol =)
Ohh boy the first four were all very good in my opinion....I stopped watching it near the end of 5 because they took it off tv, but by then they were really dragging it on, you know?
But I really liked the first four equally I think.
the Ground hog day episode is my favourite haha