It's a toss up between Shadow of the Colossus and God of War for me. I'm leaning more towards God of War since it was difficult in comparison to Shadow of the Colossus when just the normal gameplay is taken into consideration, and I like challenges as long as they don't make me want to shatter the discs into multiple pieces...which it still had its moments where I got pretty pissed, but maybe that's a good thing. I find most newer games too easy to beat and unlock stuff.

Oh, Indigo Prophecy was also pretty good.