I can't really recall what is the hardest among so many games, but these are some that left me quite deep impressions when I was playing PS1 games years ago:Parasite Eve 2 (PS1) - the freaking large boss standing in the middle of the battle area. it's just pure ugly and gives u a Game Over if you stare at his hollow bloody chest too long. I can't complete that nightmare mode, not even Disc 1 of 2 - everything's juz too hard.Vagrant Story (PS1) - everything inside during first gameplay. It was just horrible. There was no difficulty mode to set, and, everything in the game could just kill you in seconds. Things get better during subsequent gameplays coz' you have all the necessary equips and weps to take them on.Fatal Frame 2 (XBox) - after Chapter 4, things get really scary. There is not much healing stuff (coz you can't buy them at first gameplay, but find them.) and monsters are juz mobing your health away. I wonder how my sis managed to complete it once with piles of healing stuffs at the end of the game. Maybe it does not have the toughest boss, but it has some of the toughest situations.Chrono Cross - DarioI was an idiot until I read an online guide. You can't fight him head-on without some armor that absorbs damage from an certain element. I was like having no-more-than-ten-turn battles dozen times a day, seeing that freak tossing a sword that juz finished the three characters in my teams, one each turn.There are many more, juz that I can't dig them out of my brain right now.