Season 11 is good for me from beginning to end, although the first half (First 13 Episodes) has some episodes that I don't like like Pat The Horse, Bunny Hunt and Fun-Sized Friends, but it has my 2 favorites of the season which are The Legend Of Boo-Kini Bottom and Squid Noir.
Seasons 4a and 9b they are closer to the Seasons 1-3 quality as they have more smart and organic writing, however I will say I like it better when the show takes risks and tries something different because as it makes it feel more varied and fresh because I don't like it to become repetitive and boring and it's the reason why I like Seasons 5 and 11