What's your favorite?
Edit: I wonder if anyone would mind if I make a hybrid of the above form and the one beetiger indicated in comments. Xe/xir/xir/xirself. That "xem" in there does sound a little too close to "them". ("Close to who?" Shut up!)
Edit 2: Aah, what the heck, I'm not wedded to those 'X's. Zie/zir/zir/zirself is fine, and besides, no pronunciation confusion, so I guess I like it best after all.