PS1 had a lot of excellent platformers. Here are some of my favorites (one game per series):-
1- Castlevania: Symphony of the Night
2- Crash Bandicoot 3: Warped
3- Oddworld: Abe's Exoddus
4- Ape Escape
5- Tomba!
6- Rayman 2: The Great Escape
7- Spyro: Year of the Dragon
8- Gex: Enter the Gecko
9- Croc: Legend of the Gobbos
10- Jersey Devil