My opinion: Brain.

Heart, though being a bad guy and a 'Kill-All-Humans' kind of chap, seems to care for his fellow Roidmudes, and actually seems like a kind of person you can reason with, or even make pretty decent friends.

Brain, on the other hand, is a suck-up who doesn't live up to his name, being technologically showed up by another nomical hypocrite and the first Roidmude, and has a tendency to do the more evil things. And I don't like him.

I have to say these past few episodes of Drive have been stellar. Rider at its finest.

Just caught up...and well...damn...easily the show picked up when they went public with the identity. Total shift to second gear (HA!)
I do like they kind of went back to basics with the fights, using the tires again as opposed to just the forms. Why did Chase lie? No idea...but loved seeing him Rider Kick again...Now can Chaser form still use the Viral cores? That would be kind of cool to see...