I agree with the “depends on what you’re eating” crowd. But if forced to choose only one to have in my fridge, I’d go with the yellow. It’s good on more stuff. Plus, there are some things where brown just wouldn’t taste right, but I can’t think of anything where yellow wouldn’t be just fine.
Moosetard. It’s a spicy, whole grain mustard my folks brought me back from their trip to Alaska. Delicious.
Brown mustard > Yellow.
Let’s also not forget violet mustard, made with grape must/grape juice. Though that seems like it should fall under “dark.”
The two mustards I personally use the most are Plochmann’s yellow mustard and Kosciuscko Polish-style brown mustard. That said, I have about another four or five mustards in the fridge, including the violet mustard, a large grained German mustard, and a smooth beer-flavored Austrian mustard.
I think there’s also a dijon mustard and a wasabi mustard somewhere back there. Oh, and there’s also a little passionfruit mustard from Hawaii.
+1
It really depends. With a bratwurst on a bun with a bunch of sauerkraut, it’s brown mustard all the way, but on a hot dog with relish, yellow mustard is the right thing.
Sandwiches really depend too. Cheap ham and white bread (the sandwiches of my childhood)? Yellow mustard all the way. Pastrami on rye? Brown mustard for sure.
As a person of Polish extraction, this is going to bug me if I just leave it be. It should be “Kosciuszko.” Don’t know how I let that “c” slip in there. (And you can make the “s” an “ś,” if you’re so inclined.)
Man I would love a real street-vendor German bratwurst. Served on a broetchen with German mustard (senf), which is mostly yellow but very strong. Like this, it is zehr yum!
Yellow mustard like French’s is literally flavorless to me.
I find that odd, as it has a pretty clear turmeric flavor to it, in addition to the spicy mustardiness and vinegar. Some people dislike it just for that reason (the turmeric component, that is.)
Just so’s you all know: it’s amazingly easy and cheap to make your own mustard. Recipes abound online.
Mustard is a standard Christmas present from us. We make several types and include them in gift baskets to friends.
All of it. Any of it. 
FWIW I realized after posting, Brown was probably the more common name for what I meant but by then it was too late to change it.
Mustard racist.
Both
Depends on what I’m eating.
Hopefully it has more zip than that stuff they serve on klobasa in Prague, because that crap was damn near tasteless!
Since there’s one type of yellow, and hundreds to thousands of browns, the answer is obvious (not that yellow is bad, just a little pedestrian for many foods).
Tough choice, but i picked yellow.
Lots of people saying both which is great but I like polls to force commitment. 
For the record, I HATE yellow mustard (except on McDonald’s burgers for some reason) so I went with Dark. I can eat Gulden’s on almost anything.
Others are correct that different mustards are best for different purposes. However, for anything that yellow mustard is good for, brown deli mustard is better. To use game theory terminology, brown strictly trumps yellow.