I cheated hard on this adventure time card wars game. Basically they stored "gems" offline on the client, so instead of buying a few gems, I'd just set the value like gems=9999999.

The thing was that matchmaking would match you up based on how many gems you spent. It inadvertently just set us all up with cheaters. The free game was pretty meh, but cheater's hell was a lot of fun. Everyone had every card, so the only way to win was being good at the game. I'm rather sad they killed the game eventually, it was a pretty well designed game.

I think bot hell for chess would be interesting too, the game becomes who can make better bots.