It's so hard for me to pick a favorite season. I've just watched 7 seasons in 6 weeks so it's all pretty fresh in my mind but I'm having such a hard time picking one. My most favorite episode has to be Swan Song and that's in Season 5 so I guess I'll go with that.
I don't know why some people don't like S6 & 7, I personally enjoyed those seasons too. It must be so incredibly hard to keep the show running for so many seasons and still come up with something new. I think they've done a pretty good of that. And honestly speaking, if I get to see Jensen, Jared, and Misha, they have my permission to go for another 100 seasons.