Hear me out, I think It's Mr. Puzzles because the main reason is that not only he appears in the current seasons (the third half of 13,14, and the upcoming 15th season), but he is the one indirectly responsible for the destruction of Peach's Castle, which is the original location from the earlier seasons (1-12, and the first half of season 13), thus ultimately being the one responsible for forever changing the lives of the SMG4 gang.