There's a lot of good PSone games.
However, there's only a small handful of great ones that are exclusive to the platform and since so many of the PSone games are early polygonal if not all-out 3D games, a lot of the library hasn't aged well. With that said Klonoa, Crash Bandicoot, a wide variety of shmups, JRPGs and the handful of 2D games that made it to the market are mostly worth playing.
While time doesn't look back on the PSone as fondly as it does the Saturn due to the greater emphasis on 3D, don't let that deter you from playing the quality titles that are on it.
BTW the best game on the platform is Castlevania: Symphony of the Night