So far it should be kyouhei_(pokemon) and mei_(pokemon) for sure since they've been confirmed as their names. And if we're going with Bulbapedia's rendering then it's Hue, because I think Bulbepedia is trying to stick to the rivals' names' origins being associated with colors and the like. Not sure if his name really is supposed to be an English loan, but with all the other unorthodox names in the BW games, I wouldn't be surprised, so this spelling has credence.
http://www.youtube.com/watch?v=QqVU5j_yvnw is an English trailer which uses "Hugh". Someone renamed the Bulbapedia page from "Hue_(game)" to "Hugh_(game)", mentioning the video in the edit summary, apparently to justify the change.
But, I maintain what I said earlier:
If ヒュウ is originally a transcription of the English word "hue", (which I suppose we don't know for sure, but Hue seems to be our best educated guess; "Hugh" sounds too random, like "Joe" or "Paul") then the tag really should be hue_(pokemon). Official media in English, and Bulbapedia too, have a policy of using adapted names, so they would, by default, call haruka_(pokemon) "May" and ookido_green "Blue", and so on.