I would have to say our hoop. Not only did it win us the Xerox Creativity Award at the Buckeye, I used it to get out of a tight situation in one match. Because the robot has simple center-wheel drive, it can get stuck if one or both of the center wheels is/are lifted off the ground. When this happened on one of the plates stabilizing the overpass, I was able to use the hoop to push against the lane divider and move the robot back onto the track.
It also ended up being much stronger than we expected.
See for yourself.