Season 10 is my favourite, it's just too wacky, creative, ridiculous and has a amazing animation quality, and that's the stuff I like in a cartoon.
(I don't think the cartoony animation it's a problem IT IS A CARTOON)
But I objetively, season 9b is better, the storytelling is probably the best in the entire series, the plots are more interesting and well executed.
But, the only problem is the pacing, and Vincent said it, the scripts were too lengthy, so they have to compress them, and that's why the pace is very unbalanced. But this problem is fixed in season 10.