Chael Sonnen
Sonnen gave Anderson Silva hell for 4.5 rounds before finally succumbing to a submission. However, Sonnen is 34, currently not eligible to compete in MMA and has a swirl of activity surrounding his legal issues.
In a perfect scenario for Sonnen, he would be the front-runner to replace Silva upon his return, as he already has a convincing win over Okami. It seems like karma is kicking Sonnen right now, though.