Though he technically returned in Super Mario Bros.: The Lost Levels (the real Super Mario Bros. 2 in Japan), Bowser's return in Super Mario Bros. 3 was his true revival. After being bitterly defeated by Mario, King Koopa returns with his seven children to defeat the plumber in a rematch.
Even though Super Mario Bros. 3 is set in the Mushroom Kingdom, there are many kings of different lands. And Bowser and his children were responsible for them morphing into animals or monsters. I always thought that was one of the coolest things about defeating each boss was saving each king.
Bowser's Castle and his world was easily the toughest part of the whole game. Personally I think it's the most challenging levels in the whole series. I really liked how they changed the battle from the original.