Super Mario 3D World_ spoilers, if that is a thing you fear.
I am unaware of who specifically is designing the final Bowser stages for the recent Mario platformers, but either one person (or a group of people) has been doing an outstanding job or there is a challenge where someone has to step up and do a better job than the last person. The final bits of the recent Mario games I have been playing have been just great.
The Super Bell power-up in SM3DW felt weird because it would let you climb to the top of the flagpole at the end of the stage. It was easy-mode fore that, but it still interacted in the levels in a way that was untraditional for Mario games. As a cat the characters have an infinite short range attack, the ability to dive and the most important ability to climb.
Bowser himself uses the power-up on in the final confrontation, the first time I have ever seen him use a power-up in a game, and then the climb begins. Going to the top of the tower along with Bowser, or Meowser as I hope he is called, was a complete blast and required using the cat skills I had been off and on training throughout the game. While it may not have been the most difficult Bowser fight, it was a pure joy and left me smiling at the end.