raocow seems to be the biggest youtuber who plays smbx so far, though he's not your typical youtuber and he seems to be happy with where his fanbase has stabilized at. Bossedit8, on the other hand, appeals a bit more to the general youtube audience, and his fanbase has been growing at a much steeper rate. Take a look at these statistics.
Bossedit8 may not have even half the subscriber count yet but look at his growth. He's significantly steeper than raocow. As well as that, his most viewed video
has twice as many views as raocow's most viewed video
MrDragonboy96 is a similar story to Bossedit in these areas. However, if you look at the subscriber rate, projected over time:
Bossedit8 may top them both.