There aren't many mustards I don't like. I guess my preference is for the sweet grainy mustard some kebap vendors in Vienna serve on their hot dogs. I like French and Polish mustards too.

If you're a mustard fan looking for a thrill, I recommend the Napa Valley mustard festival - Calistoga in February, IIRC. You'll try hundreds, and if you're like me, you buy a suitcase full.