I think the term forgettable presidents or those who are to be labeled forgettable is somewhat disputed (and with a certain justification I think), but the interpretation of who was forgettable which I heard most frequently names all the presidents from Rutherford B. Hayes to William McKinnley as "forgetable".
apart from those two that would be James Garfield (perhaps he died too soon to be remembered for anything but incompetent surgeons), Chestar A. Arthur (to this day I wonder why the assassinator of Garfield was so keen to get Arthur into office (that wish was his motive) his previous performance as Collector of the Port of New York did not exactly cast a good light on him), Grover Cleveland (he wrote about women's suffrage: "Sensible and responsible women do not want to vote. The relative positions to be assumed by men and women in the working out of our civilization were assigned long ago by a higher intelligence." So let's better ban him from our mind

One thing that shouldn't be forgotten totally is that he kind of put an end to the dwindling of presidential power to the Congress who had cut presidential powers ever since Johnson), Benjamin Harrison (Perhaps the coldest fish ever to sit in the oval office. When a friend told him: "For godness sake, be human!" he responded: "I tried! I failed! I will never try again!" people probably didn't want to remember).
I find it kind of sad to see McKinnley listed among the forgetables and even sadder to see him remembered as the president that led the nation into the war against Spain. He was a pacifist at heart and conceded to the cries for war on Spain only when it was certain that he would be overruled anyway. More than most of his predecessors (under the strong impression of the Monroe Doctrine) he looked beyond the borders of the US.
There are more presidents who may deserve the label "forgetable" more than some of those whom I've most frequently been told to forget about.
Calvin Coolidge in my oppinion deserves mainly to be remembered for thinking that the president shouldn't do anything at all to be remembered. William Henry Harrison must be remembered for heroically defying the nasty weather during his inaugural speech (refusing to wear a coat or a head) which drastically shortened his working time in the White House.
Coolidge's predecessor Warren G. Harding is another worthy canidate I think. He was a compromise choice among the canidates and was picked because he had no enemies. He didn't have any enemies because he didn't do anything at all that would have alienated anyone, he didn't do anything that anyone would have liked either. His presidency was overshadowed by corruptcy. At least he was the first president to visit the state to be Alaska -_- .