That's a hard question...
I really like Hawaiiloa Ridge because it is short enough and easy enough that I feel very safe going by myself, and therefore I've hiked it more often than any other trail. (Except for Manoa Falls... but that's just because I live in Manoa). The views are awesome, and pretty soon you forget you're just around the corner from Honolulu.
So that's the one I've hiked the most. But other than that, it's too hard to come up with a single favorite trail.