Silent Hill - one of the scariest games (atmosphere wise) i have ever played. That radio freaked me out.
Metal Gear Solid - simply fantastic
Gran Turismo - set the standard for racing games in my opinion and the Hi-Def mode was simply gorgeous at the time
There are loads, absolutely loads of great games on the psone. It was so well supported for such a long time. The only trouble with it i guess is that a lot of the games now have various superior sequels on other newer generation formats.
There was a very good conversion of Quake 2 which is worth checking out. Dunno why the general consensus is that Goldeneye on the N64 is amazing but Quake 2 on the psone is shit. Because thats not true. The raw polygon pushing power of the Psone makes Quake 2 a smoother game.
Here's an interesting little tidbit for you. The last game released for the Psone was Fifa 2004. I'd say it's probably not worth grabbing that one if you ever decide to buy a Psone though.