The rc51 and busa/zx12 are totally different types of bikes. Vtwin sportbike vs GT bike.

Ive had both an rc51 and a Busa. I didnt really care for the rc51 (which is odd since I love Vtwins, and I ride one now.) It felt big to me, and I could never get the suspension setup the way I wanted it. And it bakes your testicles in traffic.
I really liked the busa for what it was. It has very solid feeling, had awesome brakes for a heavy bike and, of course, had insane torque. It was just not good enough up in the mountains, which is where I normally ride.
Ive never had a kawi so I dont know about that. I will say that the kawi's build quality is junk from what Ive seen, athough the chassis is a cool design. I think the busa still looks better...cool after all these years IMO Its very unique.
Id vote busa.