125's are always the most competitive. Because experience levels and bikes are basically all equal. Not even Luthi has an edge really. He's definately a contender, But I would'nt put money on it.
Probably after that is 250, simply because there's no real single standout rider since Pedrosa left.
Then I'd say Supersport. Charpentier dominates the class, but he's not without rivals.
After that would be Superbike. I got a feeling Bayliss is going to run away with this championship, but you can't discount guys like Corser, Haga and Lanzi.
Sadly, I think motoGP will be last, thanks to everyone's favourite Doctor. Sure he's got new rivals, Like Melandri, who's skill seems to be increasing exponentially, as well as new kid Pedrosa and HRC's new main man Hayden, and a very fast Gibernau on a very fast Ducati. But I don't think any of these guys are either ready or capable to be consistently beating Rossi in a way that would bring them the championship