Weisbaden.

Seriously, Mainz has good low-to-mid level food, but if you want something better it is all across the river (Weisbaden is a much richer town with a lot more tourist-oriented restaurants). If you must stay in Mainz, there is a restaurant next to the Stadttheater that has great wine and some well executed food, forget the name but it is something like Haus Deutschen Weines.