Really? In general I have a very hard time going back to PS1 era games. I really wish that I had played Xenogears back in the day because it's borderline unplayable to me now. I'm playing through BoF IV at the moment and it's fantastic and really fun, but still has weird camera issues that plagued so many PS1 games.
Top 5 or 10, maybe? I'm still trying to get into PS1 as a gamer more than a collector, as I never owned one myself (had to steal time on my brother's while he was out, or go to friends' houses).
I'm kinda biased any way, since I can go back to NES and have more fun with some of those games than current releases. At any rate, these are the games I believe still stand up:
Resident Evil 2 - Even though the N64, PC, and Dreamcast versions look better, the original PS1 game still holds up ok. The gameplay is still a pretty good balance of puzzle and action. (I havent played that much of Nemesis to have an opinion).
Parasite Eve - The soundtrack, pre-rendered backgrounds, and story really hold up. The gameplay isnt mindblowing, but it was a great precursor to stuff like Breath of Fire V battle system.
Einhander - Everything in this game still looks and sounds amazing.
Silent Hill - A PS1 game that can still manage to creep people out.
Final Fantasy Tactics - Aside from Tactics Ogre and Valkyria Chronicles, there hasnt really been an SRPG that has beaten out FFT.
Most of the games that still stand for me are, as you can see, RPGs, shoot em ups, and survival horror. I could list all FF games released among others since they deliver on well done battle systems, soundtracks, and overall story/worlds. Alot of the 3D platformers don't hold up of course, but those were just being created.
EDIT: Whats wrong with Xenogears? Havent played it myself (getting around to it).