I've been watching since about season 2ish and this season (season 12) is quickly becoming my favorite. I've been very happy with the way they have brought the story back down to the human level with the British Men of Letters plot while slowly working in the Nephilim and Prices of Hell story elements. Bring the story back down to ground level was very necessary after last season took it as high as it could go with the god/darkness story line; after that, there is really no way to escalate things any further without deescalating first.
Seasons 1 through 5 were all pretty darn good, but I will say I don't miss the emotional brother moments of Sam and Dean crying that ended so many episodes in that era. I'd say season 5 would fall near the top of my favorite list, maybe even in first or second place.
I like seasons 10 and 11 well enough, but I was very disappointed that they
ascended the fanon idea that Chuck was god. I've never liked that idea at all as it took away what was a very excellent mortal prophet character and I much preferred the idea that Chuck transcended to heaven after his job was done instead of simply being god. I also didn't like the fanon elevation because there is nothing in the previous stories that supports Chuck actually being god, the closest being
word of saint paul level hearsay by the actor who played Chuck claiming that Kripke told him to play the character as if Chuck was god.
Ignoring the backdoor pilot episode that was meant to start the Bloodlines spin off show, the worst season is definitely the Leviathans season, I think it was season 7 and some of 8, though seeing ghost Bobby was a small bit of awesome in an ocean of terrible. The pacing, plotting, scripting, directing, and to some extent even the acting was simply abysmal through out that story arc.
Favorite character is a lot harder to nail down. In the old days I would have said Bobby without hesitation, I'm a sucker for the tough love older mentor trope, but now that he is gone I would have to say it is a three way tie between Castiel, Sam, and Dean. I just love all three of them and the way they interact and play off each other. Their dynamic reminds me a lot of Kirk, Spock, and McCoy from the original Star Trek show. You have the no nonsense physical of Dean/Kirk, the emotional rationalism of Sam/McCoy, and the fish out of water logical attitude of Castiel/Spock. The comparisons between the characters of the two groups isn't exact, but the dynamics of whole are very similar between the two.
All and all, Supernatural was and still is a great show, regardless of what is said by the fake fans that trash everything after season 5. Really there is only one thing wrong with Supernatural and it has nothing to do with the show itself; the only problem with the show is that large chuck of the fandom are terrible. Besides the previously mentioned original seasons purists, there are the shippers that care more about shoving their ridiculous character pairings into the show than they do about the actual lore of the series, the whiners that complain about every episode being the downfall of the series, and, worst of all, those that cry "FILLER!!" every time an episode isn't fully devoted to advancing the season's overarching plot, ignoring that at its heart the show is built on monster of the week stories and that is where the best story and character development is done.