S4 > S2 > S1 > S5 > S3 > S9* > S7 > S6 > S8
It's pretty self-explanatory, I guess. I got to mention that the first 5 seasons are all really close. By no means did I think that season 3 was bad - it just didn't deliver as much as other seasons did, partly due to that writers strike. "Jus In Bello" was a terrific episode and there was plenty of good stuff along the way, but overall that season will probably be always remembered for its shortness and unnecessary experiments with the main cast (yeah, Ruby and Bela). While I did enjoy Katie Cassidy as Ruby, I found Bela's character to be a massive waste of screen time and Lauren Cohan's talent.
Season 8 was a disaster. I thought it's started on a pretty high note with lots of action, gore, fighting monsters with Thor's hammer and whatnot, but it quickly lost its direction and just wandered drunkenly from one mediocre filler to another. Season 6 is basically the same but it does have a few truly outstanding episodes (Weekend at Bobby's, The French Mistake, The Man Who Knew Too Much) that I would recommend as must-see. Season 8 is perfectly skippable IMO - the previouslies before Season 9 more than make up for it and will save you a lot of time.
* So far season 9 is my favorite post-S5 season of Supernatural. It's a pretty good sign, but we have to see how the season arc is resolved.