I'm presuming that melodeon refers to a diatonic push-me-pull-you box, and also to the style used on a one-row or a D/G or C/F type box. People who play C#/D or B/C I think I would call button-accordeon. One-row players who can play cajun are a bit unclassifiable, are they melodeon(English name) or accordion(Louisiana name)?
   Anyway, for mastery of the one and two rows, I stick by Tony Weatherall, for style and musical depth( he's got technique as well but that doesn't interest me vastly). And I've enjoyed Keith Chandler as well, and also a guy called Guy I last saw in Newlyn at the Smuggler (I might be wrong on the name of the pub or town, it was a while ago)..