1) KH1's Journal pointed Riku out as the Original Keyblade Master, and Nomura confirmed that by his original scenario, the instant Sora reached for Riku's hand on the Islands and then Riku disappeared was when the "physical" transfer occured.
This has Riku as the Original Master.
2) The Lingering Sentiment fight in FM+. While the fight itself is of duibous canonship, the words he spoke were confirmed as undeniable canon by Nomura when he said that in the FM+ there is a scene that again hints towards Riku being a chosen Wielder.
LS's words - when he notices Sora's Keyblade - are that Sora is not the one he chose.
LS = Terra's proxy.
Terra = was on Destiny Islands, and watched Sora and Riku play as children, thus giving him the opportunity to make the choice.
Sora = not the one he chose.
Riku = the other option we're left with, being chosen for the Keyblade by Terra, only to later lose it to Sora.