There's some amazingly weird and experimental stuff in the Japanese PS1 library, and I've been checking it out quite a bit lately. I think the sheer size of it is overwhelming -- probably another reason people don't talk about it more. But there are tons of games that are mostly or fully accessible to English speakers, and a handful of others have been translated like Policenauts and the original King's Field.
I've been sucked in by some of the more overtly oddball stuff, like Finger Flashing, Kaze no NOTAM (a ridiculously laid-back hot-air balloon game that makes Pilotwings seem like The Fast and the Furious), and the rock-climbing game Mitouhou e no Chousen: Alps-Hen. None of those deserve inclusion on a "best" list, some may not even be good games, but they're surely distinctive and unusual.
The PAL/European library has some really interesting games as well -- I'm just starting to check that out, though there's some real garbage too (including a whole series where they paired hilariously bad ripoffs of recent animated movies with a jigsaw puzzle and a coloring book, and called the result a game).
I wish we had a complete list of PS1 games that were only released in both Japan and PAL territories, with notes describing how well the PAL versions run at 60Hz. Some can only be played in English via the PAL version, but 60Hz compatibility is an open question. There's at least one game that's only in Japanese and Italian (Yattaman: Time Bokan), but I don't know if any others were translated to another European language and not English.
BTW I've already found one PAL game (Actua Tennis) that doesn't work at all with ImportPlayer or ImportPlayer Light -- it plays the intro FMVs, but hangs at the title screen -- could be anti-piracy, I suppose. Sadly there's no NTSC-J version and my PAL>NTSC converter will only display the image in B&W unless I get a real PAL console.
