Messiah. Probably not that hard to beat I suspect if I put my mind to it now, but for reasons lost in the mists of time, I couldn't work out how to beat the final boss. For those unfamiliar with the game I'm talking about, you basically played Bob an angel (though more like a cherub) who gets sent down from Heaven by God to a world that's gone to shit in a dystopian fashion under the influence of the devil. Your only power is the ability to possess the living and utilize their particular skill sets to help you progress, often times with hilarious consequences.
Actually a pretty neat game (I really dug the visual style of it), more puzzle solving than anything else, but the final boss fight turns into a bit of a melee with you having to move from host to host fighting the Devils own fallen Angels as well, and I don't think I had the tactical smarts for it at the time.