I'd strongly prefer doughnut.
Posted under General
Wikipedia said:
Doughnut is the more traditional spelling, and still dominates outside the US. At present, doughnut and the shortened form donut are both pervasive in American English. The first known printed use of donut was in a Los Angeles Times article dated August 10, 1929. There, Bailey Millard jokingly complains about the decline of spelling, and that he "can't swallow the 'wel-dun donut' nor the ever so 'gud bred'." The interchangeability of the two spellings can be found in a series of "National Donut Week" articles in The New York Times that covered the 1939 World's Fair. In four articles beginning October 9, two mention the donut spelling. Dunkin' Donuts, which was founded in 1948 under the name Open Kettle (Quincy, Massachusetts), is the oldest surviving company to use the donut variation, but the now defunct Mayflower Donut Corporation appears to be the first company to use that spelling, having done so prior to World War II.
So yeah, who knows. I don't mind either one. By the way, we do usually prefer American spelling over British, right?