Trigger buttons are interesting. They face away from the player, you don’t look at them, they’re identified loosely by shape.
Glancing at a control prompt, a shoulder button (L1 or R1) is flatter, and the trigger (L2 or R2) is a taller shape.
I really like the PlayStation 4 controller.
Its triggers have well defined shapes, that also fit nicely into a square – which is convenient when making emoji-like control prompts.