Not quite an accordion. But the best player I've heard in recent times for that sort of instrument is Sharon Shannon.
If you're into Irish contemporary and trad her full concert at Dolan's warehouse is (currently) up on YouTube.
Sharon is a great player of the button accordian. I play in a couple of Irish Sessions every week (tenor banjo and cittern) and at one we usually have two or three B/C box players, one or two C#/D box players, maybe one piano accordion player, a few concerterna players, and the usually array of flute, whistle, fiddle, and pipers. I had the great opportunity to play with Sharon and her brothers just as she was emergining in the media.