Yes, Sora is a chosen keyblade master. He is the main Keyblade wielder, the second being King Mickey. Sora was chosen to be the keyblade master because of his powerful heart and strength.
SPOILER******************
Terra wanted Riku to be the keyblade master, but Sora's heart was more powerful, thus Sora was able to combine his heart with Ven, allowing him to wield any keyblade.
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The Keyblade chose Sora in Destiny Islands. Sora's first Keyblade was the Kingdom Key, but it changed into different versions, like Hidden Dragon, Circle Of Life, etc.
probably Sora but Riku was supposed to be the chosen wielder until he gave into darkness in KH1
Roxas was created when Sora stabbed himself with a keyblade. So, he is Sora's nobody.
aqua is a character from the PSP game Kingdom Hearts:Birth By sleep where she takes the Master Qualification Exam along with Terra and becomes a Keyblade Master.
Well, if you were to decide the best keyblade by stats, then that would probably have to be the ultimate keyblade, which is the very last keyblade you can obtain and must be synthesized with the moogles. But if you were to decide the best keyblade by its special power, then that would be up to you, as to whether you like lucky or more mp or more defense.