Dan Henderson "could" beat him due to his style of fighting. The problem is when he cuts down to 185 (he's 205 now) he gasses quickly (see Jake Shields or the first Silva fight). If Silva moved up to 205 that would be a damn good fight.
Sonnen had him beat too but got caught. Not sure he would beat him in a rematch but I would like to see it.
Machida or Rua would give him a run for the money at 205 but they are training partners and Silva has said he would never fight him. GSP I'm not sure could beat him either but he would have to move up to Silva's and would hold him down and it would be a boring fight most likely (or GSP would get knocked out).
The only person I think would give him the most/best fight is Jon Jones. I would love to see that one