My top mustard is Mustard de Maille's whole grain dijon. Tasty stuff.
For brats and such, Inglehoffer stone ground german mustard
For general use, Grey Poupon Dijon.
Gulden's Spicy Brown is good too, but I usually let the Grey Poupon do the job as it's on hand and I don't use enough to make it worth having another mustard around.
For chinese, Colmans ground mustard and some water or rice vinegar mix up nicely.
I'm not fond of the yellow ball park style mustard. I've seen a few recipes that say only that kind works for that recipe but I haven't tried those dishes