Wow, that's a hard one, there are so many good ones!
Hmm, I'll say Quake, Final Fantasy VII, and Metal Gear Solid.
Quake because it was Doom on steroids and for it's time was absolutely amazing.
Final Fantasy VII because it's Final Fantasy VII
and Metal Gear Solid because I loved the original and to take the original and then be able to do everything you could on the PS1 version made it all the better.