More importantly than the glass (I love Schott glass...) is the type of ring. Tiffen and other cheap filters use aluminum and can bind to the lens (especially if your lens is the cheaper kind with plastic threads). B+W and Heliopan use brass. A lot better.
Realistically, the Heliopan MRC is the best filter. Even better, get a Heliopan AND a lens hood. A REAL lens hood, not the useless "flower petal" kind that come with lenses.
Makes one hell of a difference.