What password did Neville Longbottom forget?

Answer 1

All of them. Neville Longbottom could only ever remember the first password used by the Gryffindor common room in his fifth year because it was the name of his plant, Mimbulus Mimbletonia.

In Harry Potter and the Philosopher's Stone he notably forgot the password "pigs snout" and was sleeping on the stairs after being locked out overnight. That is until Harry, Ron and Hermione found him on their way to a Midnight Duel.

In Harry Potter and the Prisoner of Azkaban, Neville wrote the weeks passwords down and then lost the piece of paper, which was actually stolen by Crookshanks and given to Sirius Black. After Sirius used it to enter the common room, McGonagall banned anybody from giving Neville the password and he would have to wait until somebody let him inside.