Depends on what you're counting by "Super Mario Brothers"
Whole franchise - Count Bleck (though really I love him and Rosalina, Dimentio, Fawful, Luvbi, and TEC-XX so much that it's hard to choose one favourite)
Main series - Rosalina, as the only member of the above list who isn't RPG exclusive
Super Mario Bros. original trilogy on the NES - Bowser
Super Mario Bros. 1 - also Bowser
I'm not a particular fan of any unofficial characters. If I had to choose one, it'd be Bowsette, but I like Bowser more than her.