Good PS1 platformers ? Ape Escape, Klonoa, Medievil, Oddworld, Skullmonkeys, Tombi series, Blasto (it's quite hard) and Croc 2.
For something quite underrated (not just platformers) for the PS1, here's a video about it, it has some good underrated games:
Hey don't click here ! It's creepy !!
I want a PlayStation All-Stars Sequel ! Come on Sony, stop making too many shooters !
I'll make a better signature when I'll have time to do it...