For me it is an abomination known as "Hand Simulator" and it was nearly the death of me
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Mr and my brother couldn't beat the final boss in Sonic Unleashed for about a week and it was very rage inducing but there was a treasure chest chase in Rayman Origins and it was one of the most nigh impossible things I'd ever done in a game. It was extremely frustrating and I'm not even sure if we ever beat it....