Donut. Doughnut is too formal. And Firefox says it's wrong.
Chrome says Doughnut is right and Donut is wrong. Interesting.
Apparently I originally voted Doughnut on this, but I'm not so sure anymore. Donut looks wrong, but of course, any word stops looking like a word when you spend so long analyzing it. I guess it just looks too similar to Don't?
The thing about Doughnut is that having Dough be recognizable in there starts raising the question of where the Nut comes in. Reminds me of the Action Button reviews about how the more Metroid Prime 3 and Bioshock try to make sense, the more obvious it becomes that they don't make sense.
Turns out it's actually an interesting turn of synecdoche. The original doughnuts were actually nut-shaped balls of dough. Later on, the ring doughnut and filled doughnut became more popular, and somehow or another the ring doughnut ended up being the one popular enough to drop the adjective, and then 200 years later, the original doughnuts are called doughnut holes. Knowing the history makes things make more sense, as I'd always wondered why jelly doughnuts counted as doughnuts when they weren't toroidal.
So doughnuts have been around for about 200 years, and the "donut" spelling has been around for at least 111 years. Not nearly as much of a neologism as I'd expected.
By the way, Chrome's spell-checker doesn't recognize "synecdoche" so it is officially dead to me.
Anyway, now I'm starting to prefer Donut because of how many times I've typed Doughtnut and had to go back and fix it. Typing Donut flows so much more smoothly.