To see how quickly you can type the last names of all U.S. presidents, go here. The time limit is 10 minutes. I finished with 7:53 remaining; that is, I did it in 2:07.
Entering each of the names Adams (John and John Quincy), Harrison (William Henry and Benjamin), Johnson (Andrew and Lyndon), Roosevelt (Theodore and Franklin Delano), and Bush (George H.W. and George W.) accounts for two presidents. Cleveland, who served two non-consecutive terms, is covered by entering his name once.
The most "neglected" presidents -- those who have been named only 49 or 50 percent of the time by more than 88,000 "guessers" (as I post this) -- are (chronologically) Fillmore, Pierce, Buchanan, Hayes, Arthur, and Harding. Andrew Johnson probably would be in that group were it not for Lyndon Johnson.
The presidents most often named have been Bush (94 percent), Washington (93 percent), Clinton (90 percent), Lincoln (89 percent), Kennedy (86 percent), and Nixon (85 percent).
It rankles that Clinton is named more often than Lincoln (if only slightly). It is consoling that the Roosevelts (at 84 percent) do not outrank Washington or Lincoln.
That only 93 percent of the entries have named Washington is a testament to the low estate of American education and/or the vast geographical reach of the web.