Serious Eats - seriouseats.com

I like a lot of different kinds, all mentioned above, but wanted to put in a vote of confidence for the Garlic Dijon Aioli at Trader Joe's. It's goood.

-Ben

My work's home base is in Switzerland. There, I discovered Thomy’s Senf. I thought at first, it was toothpaste, as it's sold in a tube (very portable). Thomy is a Swiss brand that produces food products such as mayonnaise, mustard, salad dressings, and the best [again in a tube] tomato paste. If you can get a tube of this Swiss equivalent to the U.S. 'every home has some' of French's Yellow, you'll love it. It's creamy and thick, has a nice sharpness and is packed with amazing flavor.

Raye’s “Down East Schooner” Mustard. It was also the gold medal winner in the 'Classic American Yellow Mustard' category at the 2009 Mustard Competition.

Maille regular and whole grain Dijon
Gulden's Spicy Brown Mustard
French's Yellow

Bertman's Ballpark Mustard - absolutely the best! (I live in CT but order it from OH)

Stonewall Kitchen's Blue Cheese & Herb Mustard, amazing flavors abound.

Maille Dijon, like many others have said. Though I also have developed a fondness for Grey Poupon coarse ground mustard, which I often use in my cooking as well.

Kosciusko Spicy Brown Mustard

Yellow mustard doesn't taste like mustard to me. It tastes like Vinegar.
Maille's my brand for sure.
@Expatslat - oh man it's a deadly blend!! soo good
I'm a fan of most President's Choice condiments as well.

Ditto Coleman's, but for everyday I like the Natural Value yellow mustard that's cheap at my co-op

Pommery Green Peppercorn Mustard, or, being all chichi - Moutarde au Poivre Vert. This stuff is so good I'll just eat a spoonful of it they way one does with peanut butter. It's not cheap, but man, I go through it like like wildfire.

Raye's Sweet & Spicy mustard is my new go to sandwich slatherer, I found it at World Market here in Florida and will stock up incase they pull it. My back up mustard is Ben's Sweet Hot. Both are outstanding full-flavored condiments I use for sandwiches and as additions to my feeble attempts at making sauces.

Honeycup mustard. Vermont Maple Horseradish Mustard, which appears not to be available separately on their web site. French's classic yellow on hot dogs, since the hot dog wouldn't taste like a hot dog to me otherwise.