Ninja Gaiden...by far. If I would have not beaten that last dude after my 49th try...I would have thrown the controller into the TV.
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Pandora Tomorrow is easy. I beat it at a friends house for crying out loud. Last level is kinda annoying though, took me like 15-20 tries but I finally got the ND133. :-) Hardest game? Try Marathon on Total Carnage.