Horseradish mustard - Weber's brand from Buffalo. Both
mustard and horseradish seem to have the same aromatic
non-pepper hot chemical that also appears in wasabi, so
a horseradish mustard can be a lot more intense. Weber's
brand is fairly mild among brands that combine the two
ingredients.

Dusseldorf style stone ground - It has visible pellets in it
so it is just a little chewy.

Cole's powdered mustard in the yellow can - Reconstitute
with water to your own intensity level.

French's cheap yellow in the squeezable bottle - Because
sometimes it's about convenient not about gourmet.

I've read that mustard is closely related to the cabbage
family. Huh. So if I bite into a raw brussel sprout and it
burns, that's the same chemical in another plant? Maybe.

I know that with hot-pepper hot folks build up a tolerance.
I don't know of anyone who's claimed to build up a
tolerance to mustard/horseradish/wasabi hot over time. I
know folks who have different tolerances but it seems like
they've had that tolerance as far back as they can remember.
It's got to use a different mechanism than the hot from
peppers. It sure feels different with that flame-like rush up
the nose.