What's your favorite mustard plant variety? I'm especially interested to hear about those with large leaves.

The one I use the most of, and grow a cluster of it in the off-season in hydroponics, is Mizuna. It's the smaller leaf type, that keeps coming back over and over, has a mild flavor, and is good in salads, as well as cooked dishes. This red one is the only red one that had lasted this long w/o bolting. I've had other reds bolt much earlier, while greens have lasted well into July, before bolting, and into April or May, in the hydroponics.
Beni Houshi Mizuna, 5-27, with no bolting, and growing like crazy! Most reds bolt early. by pepperhead212, on Flickr

A bunch of the red mizuna cut away, showing the leaf lettuce and some later planted green mizuna. by pepperhead212, on Flickr

Komatsuna is a favorite larger leafed one, but senposai (komatsuna x cabbage) is even larger, and longer lasting, and I've had misome (komatsuna x tatsoi) last even longer. And all of them have relatively mild flavor, for mustards.

I like the hot curly leaf mustard I used to be able to get in the store.
Haven't seen it for years.

Here's that red mizuna again, about 2 weeks after that last large harvest. I've cut bits and pieces from it, for salads, but it was totally growing over the lettuce again, and was even larger, as you can see. No sign of bolting, which is great. And the green mizunas are taking off, too, as you can see! And all these clumps started as one plant each.

You can see why these are my favorites, overall!
Less than half of the single red mizuna plant harvested at one time again. by pepperhead212, on Flickr

Red and green mizuna, along with some leaf lettuce. 6-11 by pepperhead212, on Flickr

The largest of the green mizuna plants, 6-11 by pepperhead212, on Flickr