Silent Hill is a must play game, it's the most immersive Ps1 game, I used to spend long hours chasing clues on top of feeling terrified. Gran Turismo 2 used to be my favorite by far followed by Crash Bandicoot racing.
Other fun stuff: Vagrant Story, Legacy of Cain, Vigilante 8, Valkyrie Profile, Ace Combat 2, Driver, Resident Evil 2