Popsloader
may not be needed... but it depends on what firmware version you are running. Each firmware revision had changes to POPS to improve compatibility... this was certainly not always the case.
Hence why popsloader was created by DA... to allow you to run PSX titles on your
PSP with previous versions of POPS that are possibly more compatible than the following versions.
If you are asking what games you can play without popsloader, you will also need to tell us what firmware you are running. Even then, the game may play without popsloader... just not very well.