I'm a big fan of mustard. Dijon mustard has a specific taste that you won't find in other mustards. This isn't to say Dijon mustard is the best (best is in the eye of the beholder), but it has its own taste.
Personally, I grew up with Amora "Fine et Forte" (fine and strong) Dijon mustard:
So if you ask me, nothing comes close, and that's the real thing - probably just because that's the taste I was accustomed to in my youth. Nothing like that specific mustard, the first day you open the lid. It's super strong, and will make anyone not used to it choke. I had an Indonesian friend who could eat super strong chilis and crazy amounts of horseradish, but when I gave her one spoon of the Amora fine and forte, she became all red and sweaty,
Different kinds of heat I guess.
In my house if you opened the lid to get some mustard and left it open, you would get yelled at: if you don't close the lid immediately after using the mustard, the heat of the mustard slowly disappears.
I haven't been able to find any mustard that come close to it in the US. Not the Maille, not the Grey Poupon, not the Trader Joes. They all taste good in their own right, they just don't have the strength, the heat of the Amora Fine and Forte.
To this day I still have my relatives bring me that mustard from France any chance they get.
Otherwise the closest I have found to the Amora in the US was the Trader Joe's unassuming mustard jar, it's got a white label.. I wish I could have found a photo. They don't always carry it, so when they have it I buy 4 or 5 jars. It's not the same, but it's the closest.
And not to say that that's the standard way to name things, but back in France, when we said "Moutard de Dijon", that would usually be, by default, a very strong smooth mustard, without the grains. The one with the grains we'd call "Moutarde a l'ancienne", and for that we'd usually buy the Maille - but really it's a different animal, that doesn't come anywhere close in heat.
If I were to hold a mustard tasting, I guess I would serve cold pork roast sliced kinda thick, or a good cooked ham, and good fresh "neutral" white bread, like a baguette, a white country bread, or maybe a focaccia or ciabatta.