Honestly, I thought he stopped being the best character after Days. Don't get me wrong, I still like him, and the prologue section with him in KH2 is still one of my most beloved moments not only in Kingdom Hearts, but in gaming in general. But I feel like the later games knew how popular he'd become and milked that popularity a bit too much. Same with Axel/Lea.
To me, the greatest aspect of Roxas' story was his tragedy. I'm fine with them bringing him back, but to me everything that made him interesting was wrapped up in the tragedy of his circumstances. Once the tragedy was removed, he's basically just a dual-wielding slightly grumpier Ventus. I'm glad to have him back, but I don't think he has much staying power anymore beyond an occasional assist to the main characters.
Right now, I think the most interesting character is easily the Master of Masters, partly because of how mysterious he is, partly because of how powerful he seems, and also in large part because he has such a unique personality for a villain. He's such a goof and incredibly playful, but you can still get hints of a sinister malevolent mastermind beneath the mask. I've also really grown to appreciate Xehanort's character more and more over time, even though I do still think his characterization hasn't been handled perfectly. And Riku has ended up having one of the most dynamic and poignant character arcs of anyone in the entire series.
I like Roxas, I just think he's outlived his moment in the spotlight. He had his crucial role in the narrative, and now it's over. His arc is complete. I just don't want to see them constantly shoving him and other fan favorites in our face all the time, because that's the quickest way to make audiences sour on those characters.