My current rankings during my rewatch of Foster's Home for Imaginary Friends, now including all of Season 2 which I finished a few days ago, as well as the episodes from Season 3 that I won't be rewatching (which unfortunately includes the first of several Z tier episodes, and one that I've always hated even when it first premiered back in 2005).
Some say that everything after the pilot sucks, others say everything after Season 1 sucks, but where I fall is more on the side of everything after Season 2 being a mixed bag. Seasons 3 & 4 are the point where I felt the Seasonal Rot really kicked in with one of the biggest factors being the very idiotic decision that Cheese (the very definition of a one-off character) needed to have more episodes where he played a pivotal role, all of them awful (most of which make up most of the episodes in Z tier)! And considering how low his debut episode Mac Daddy is currently just goes to show how he's a character who only barely scrapes by as a main character one time before he turns into an unwanted annoyance afterwards.