Mine is either Bowser Jr., Mario himself or Rosalina :3
Bowser Jr. Since I find his personality and characterization endearing, his paintbrush abilities to be awesome, and generally an awesome character compared to Bowser's previous "children", the Koopalings.
Mario since he's pretty much the go-to gaming icon, a blank slate of a character with many different and fun mechanics no matter the game.
Rosalina thanks to her simple, but awesome design, sad and deep backstory, her star theme (I'm an astronomy fan <3) and the mysterious aura she gives off.