If you ask for stardard shumps, 2D scrollers but some with 3D graphics, here are some in no particular order:
Einhänder
R-Type Delta
Raycrisis
Raystorm
G-Darius
Darius Gaiden
SDF Macross - Do You Remember Love
Gradius Gaiden
Donpachi
Dodonpachi
Gaiaseed - Project Seed Trap
Raiden DX
Salamander Deluxe Pack Plus
Harmful Park
Soukyou Gurentai
Thunder Force V - Perfect System
Anyway I consider Einhander and R-Type Delta as the best PSX shumps