i second tomba, gunners heaven, and harmful park. you can get gunners heaven off the japanese psn store now.
chippoke ralph no daibouken (or the adventure of little ralph) is one of my favorite ps1 games -- it's an awesome arcadey platformer with great level designs and tons of little secrets and charm to spare. and it's worth playing for score/1cc. uh...and it's super fucking awesome
also zanac x zanac -- remember compile, who made musha and space megaforce and, you know, ZANAC? well this is zanac plus a NEW zanac game appropriately called zanac neo and it's awesome and compiley and the effects will literally hurt your eyes
both of these games are fuck expensive, it occurs to me. oh well.