King Naruto said:
Wrong and your arguement sucks. Sephiroth used his full power against Sora. When you finally get him down to low health then Sephiroth pulls out all the stops to try and beat Sora but, in the end Sora wins. Sephiroth is gone.
Xemnas is nothing really. In the final fights with Xemnas, it's basically Sora fighting Xemnas. Riku really didn't do anything except for when he got Sora out of that attack. Xemnas is gone.
Riku could beat Sephiroth. Riku is actually a stronger fighter then Sora. Sephiroth would fail in a battle against Riku. Sephiroth's katana is no match for Riku's Way to Dawn keyblade.
Xemnas can't beat Riku as well. Riku is way stronger then Xemnas.
Now for the real battle. If Riku and Sora fought eachother at full strength then Riku would comeout the victor. Riku lost to Sora in the first KH because it was actually Ansem taking control but, Riku is way stronger then that. Now that he has his keyblade and he has gained power like Sora. Riku would win. He has more skill, and more talent. You just don't get to see him at his fullest potential.
The Answer appears to be Sora (because you said Xemnas, not Final Form Xemnas, in which case I would have gone with Xemnas) He beat Seph, he beat Riku like how many times now.
Hollow Bastion
Hollow Bastion with Xehanort controling him
Destiny Islands with Xehanort controling him
Kingdom Hearts with Xehanort controling him
Kingdom Hearts again with Xehanort controling him
Land of Dragons with form of Ansem
The World that Never Was in the form of Roxas
Ok, he has beat Riku 7 times, one of which Riku had the keyblade which he was destined for, but his heart was weaker than Sora's so Sora got it back. And of course he beat Xemnas one on one. so the order would be
1 Sora
2 Riku
3 Sephy
4 Xemnas
However, if Final form Xemnas is thrown into the equation, everything changes.
Final Form raped Riku at the very end of the game (right before Riku gives the keyblade to Sora) and he would have killed Sora if not for Riku (during the "Can you spare a heart?" move)
If he did not have to fight 2 keyblade masters, Final Form Xemnas would have won. Even Riku at his full strength was not as strong as Sora (demonstrated at the end of the game as he gets raped, and can barely stand, and even admits that Sora is better than him.