Our robot
One open-source design, one parts list, one shelf of spares that fit everybody's machine. When your robot does something strange, four other people have already seen it do that.
It's free, it's documented down to the print settings, and people who aren't us have won with it. That combination is rarer than it sounds.
It also takes a bolt-on spinner without redesigning anything, so the club can argue about weapons later without throwing away the robots we built first.