Algodoo

I wouldn't say that was perfect. You'd think it would blow up when it hit something, but did nothing at all.

The fact that it shoots straight for so far and can hit a wall like that from so far away is what's so perfect about it. I've only ever built two things accurate enough to hit that wall regularly from that distance, and neither of them are rockets. One doesn't work in version 5, and the other would probably need some modification to shoot that far.