This is a relationship based question. my best friend has a girlfriend and i have a crush on him. should i wait until they break up to confess to him?

Tough question, but: It might be wise to wait, since your confession alongside his involvement with someone else could likely complicate things in the friendship. It might come down to whether or not the relief you might feel with the confession would outweigh the risk of him potentially being uncomfortable knowing about your feelings when he's already committed to someone, and subsequently push away from you.

Personal decision: If he is your best friend you should know him well and the likelihood of his response. You shouldn't place him in a position where, if he is not interested in a more intimate relationship with you, that he feels it necessary to terminate the relatlionship. On the other hand, he may feel the same about you and afraid to voice it. Only you can judge, but the girlfriend should not deter you.