At middleweight no one's really gonna offer much. Guys with the best skillset to beat him are really only Vitor because he might have better boxing and is fast as fuck, Hendo because he's Hendo and won the first round when they fought, Jacare because he's one of the best BJJ guys on the planet (better than Maia so I left him off) and looks better standing up every fight, and Manhoef because he's probably P4P the hardest hitting guy in MMA/K1. That being said, he already beat Hendo, he'd probably beat Vitor in the clinch, if Jacare couldn't get him down he'd get KO'd and Manhoef's way too wild for a counter guy like Anderson.
It's a little tougher for him at light heavy because guys are hitting harder. Since he won't fight Machida or Lil Nog the other two with the best shot at beating him are Shogun and Mousasi. Shogun has Silva's muay thai but doesn't have his boxing, and vice versa for Mousasi. Both are also comparable with him on the ground. If Arona can ever get back into fighting he'd be a nightmare for Anderson since he can hold anyone in the world down for 15 minutes. And if Jardine fought him beforehand then Rashad could have a chance too. I think Rampage loads up on his power shots a little too much to be able to catch Anderson, but he's got a decent shot as well.