Weakest or strongest of the three?
My order is
The Amazing World of Gumball
Regular Show
Adventure Time
The show used to be interesting for me, but then it started going stale. I feel like the show is going south with only a few good episodes. The other two shows have yet to have a bad episode in my opinion.
I can say the same with Courage the Cowardly Dog (I'm a huge fan of the show); After repeated viewings of all 54 episodes, the show goes south with only a few deserving repeated views.
Comparing Courage the Cowardly Dog to lets say, Dexter's Laboratory, Dexter is somewhat ahead of Courage.