Alohamora

Alohamora is a spell to unlock doors which works nearly every time.

The first time it is used is by Hermione in Harry Potter and the Philosipher's stone by the door guarding Fluffy's door. It is used later in the series often by Hermione but also by the Death eater's and Harry attempts it to open the Horcrux.

The only time's the spell seems to fail is when Harry can't open the Horcrux and by Hermione when she can't open the door in the Department of Mysteries.