I've been watching this stuff for a while now - which doesn't mean I know anything, but I know what I've seen. While I'm a GSP fan, I've gotta go with Silva as the best right now. He just makes things look so easy.
That said, Fedor Emelianenko is freakin scary good. I don't see a heavyweight out there right now who'd stand a chance against the guy, but I thought the same about Crocop and he fizzled out when he came over.