I'm going with Machida. Considering the level of competition he's already beat, he's no fluke. And even though the other divisions are considerably weaker, I think Machida can run through the superstars at light heavyweight. Silva has a long streak going, but moving up in weight and back down will fuck with him I think. St Pierre is becoming a typical ground and pound fighter and I think he'll slip up against Alves. Bj Penn is good enough to dominate his division, but he is also on his way down and doesn't train enough for me to pick up. Lastly Lesnar just isn't experienced enough. Hasn't proven he has a chin, and is weak against the submission. And personally I think Shane Carwin or Chieck Kongo would run his show and they'll both get a shot soon enough.
Machida's style is too different for people to adapt to and the fact he will wait out the entire fight for his opponent to leave an opening makes me think he will have his title longer than the rest.