My absolute fav is Swedish Slotts Senap ("castle mustard" lol). Of the various varieties I know of, Skånsk Senap (from the south) is just fantastic on coldcuts, ham in particular! Sweet ~ hot ~ grainy and a lot of body. Mustards are actually quite easy to make if you can find the various seeds;
with the exception of a good French Dijon which I use for vinaigrettes and sauces ~ couldn't possibly even begin to try duplicating them.