Masters in KH at the time of the latest chronological game (that I remember):
King Mickey
Master Yen Sid
Aqua
Terra (I'm sure that he proved that he's a master in his own right, but I can't really recall.) + Master Xehanort
Masters I think are likely to train Sora if he ever gets trained ('cause I kind of doubt that he needs it after the adventures he's had):
King Mickey
Aqua
I don't think Master Yen Sid would train him, because he had his chance to discuss an opportunity to train him. I mean, when they first met, he could have mentioned that after this adventure, he would request Sora to meet him again for training. He didn't mention anything of the sort, so I doubt that Yen Sid will train Sora.
Uh, I shouldn't have to explain why I think MX won't train him, but I'll note that I doubt that Terra will want to train him after years of emotional struggle with MX. Also, there's a greater chance of Terra and MX dying at the end of KH3 than the two of them being separated and Terra being saved.
I don't understand why people think that Riku is a keyblade master. He's had a keyblade for less time than Sora has, and Sora has gone on more adventures and fought more foes than him (looking at the games, that is). I'm not saying that Riku doesn't have any adventures under his belt; he traversed the Realm of Darkness with the King after KH1, after all, and he was in Castle Oblivion when Sora was.
But like I said, I think that either King Mickey or Aqua would try to train Sora. In regards to King Mickey, he is a master already, and he has aided Sora in battle before (though I'm unsure whether those instances are canon or not). The King could train Sora for a time in Disney Castle, or the King could go to Destiny Islands and train him. Some issues with King Mickey training him would be the fact that King Mickey is a king, and he must have more pressing priorities than training a kid to becoming a master, and it doesn't take only a few years (look at Ven, who was "too young" to take the test to become a master).
In regards to Aqua training Sora, I believe that she would be the best choice. Think about it, she doesn't have responsibilities on a specific world; her biggest priorities would be helping her friends, and what better way to do that than to train the kid helping her? She's the only character out of VAT to be officially declared a master, and she's obviously willing to sacrifice her time to ensure her friends' safety.