I tried 30 mustards this summer
Not a bad survey of American mustard, but still missing several entire classes of mustard:<p>* French <i>moutard a l'ancienne</i> ("ancient" style), with mustard seeds left in.  Maille's version is my go-to.<p>* Japanese <i>karashi</i>. Extremely spicy, so a little goes a long way, but a must for some Japanese dishes like <i>oden</i>.<p>* German mustards!  Sweet Bavarian <i>senf</i> is obligatory for white sausage (weisswurst).