Playing on a phone is tough for PS1 games that use L and R buttons, you should get a gamepad for it, with an OTG cable or a Bluetooth one (I have one).

Anyways, my all time favorites are:
- ✔️Crash Bandicoot series (the second one is the best for me)
- ✔️MediEvil I & II
- ✔️Heart of darkness
- Ace Combat 3 - Electrosphere
- ✔️G-Darius
- ✔️R-Type Delta
- ✔️Rayman 2 - The Great Escape
- Oddworld Series
- ✔️Donald Duck - Quack Attack
- ✔️Wipeout 3
- Apocalypse
- ✔️Spyro series 

The one with the check ✔️ are the easier to play with just the on-screen buttons.

Also, you might want to try other platforms like SNES, GBA too, they're much more friendly to play on a phone and have much smaller ROMs too. 😎