Mr Wordy
CalifJimWhat's the most expensive restaurant where you have ever been?
You think this is OK then? That's curious; to me it sounds quite wrong.
I consider it bad as well. The problem comes from where and been in the same clause and the fact that a short visit is implied. Where is equal in meaning to in which:
This is the house where he lives. = This is the house in which he lives. = This is the house [that/which] he lives in.
If we use where in the original sentence, we get:
What's the most expensive restaurant where you have ever been? = What's the most expensive restaurant [which/that] you have ever been in?
Which shouldn't be used because of the superlative the most expensive but even with that the sentence seems a little odd. To is normally used with been to indicate a short visit.
CB