PS1 library is huge, varied, and has fantastic games in EVERY category. I know a lot of people focus on the RPGs, and there were fantastic RPGs in the PS1's library, but don't overlook the other genres, either. I'd recommend just jumping in the deep end and picking out stuff that appeals to you. Even a lot of the cheapies are good.
There's just so much out there, it's hard to make post about what games are good on it, in general.
What kind of genres are you into? That might make things a bit easier.