My rank of SPN's seasons would be: 4, 2, 1, 3, and 5.

I didn't bother watching season 6, because I fell off the SPN bandwagon during season 5. I just thought the show became too inconsistent in tone, and suffered because it was being pushed to continue past the original five year plan. And I'll be honest - the way female characters were often treated plus the constant Sam and Dean angst that just never seemed to change or create any character development of the brothers got old for me.

I think season 3 would have been better if not for the writer's strike.