Personally, distilled/purified water lang ako. Before, I used to put a drop or two of Joy Ultra, but since I started waxing the car, those small drops end up making water spots when they dry out.
So now, distilled water lang ako, because if you clean windshield regularly, no real need naman for concentrates.
And, oh, yeah, Rain X na rin (but not the one you dilute in with water -- I like applying it directly to windshield).