could be:

bent triple(s)
mis-aligned forks/trees

or the obvious, a bent handlebar.

if you have some way of locking the front wheel straight (like a bike lift) lock it down, make sure the forks are both even in the upper triple, loosen the LOWER triple pinch bolts, and yank the front end straight by pulling the bars the way they need to go. Once it's centered, tighten the lower triple pinch bolts back down. If you tighten them after straightening the front end and it moves back to being crooked, you probably have a bent triple clamp.