Ah, Ape Escape and the duel shock. For external hardware, perhaps the vast controller library. Not only the standard controllers or various arcade sticks, but all the unique driving (NEGcon/Jogcon), Flight controllers (PS1 Duel Flightstick), train controllers, music controllers and the forgotten Nyco Trakball. That Trakball perfect for PS1 Tempest X3. The PS1 with so many controllers; the standard for wide range of game controller adapters for other systems. Buy the huge expensive arcade stick for the PS1 and pretty solid there will be an adapter available for most other consoles.
Video Card
As far as max video push on the software side, there are some games that supported the higher resolution 640x480 mode; Tekken 3 and Internal Section for example.
Two Player Split Screen is nice - Two Player Two Screens are better
Another forgotten mode are the Link Cable games requiring two PS1 consoles and two video displays. Assault Rigs, Command Conquer and other games; the PS1 has a small library of Link games. Local multiplayer adds to the fire over online play.
https://en.wikipedia.org/wiki/PlayStation_Link_Cable





