Been looking to pick up one of those PSX adapters.
I have tried:
Competition Pro: Very accurate but a bit stiff, no autofire, no second button
Gravis gamepad: The amiga versions of these are a bit rare but its great. 2 buttons, autofire for both, and an option to change the autofire buttons to be "up" and "down" so you can play platformers console style.
Bonus that the autofire is by holding down a button, rather than a switch
Comp pro CD32 controller: I have one, but its really worn out. Too bad, as its pretty wicked. Autofire for all the buttons (with 3 settings). Unfortunately on mine, the primary fire button has to be pressed really hard
Genesis controller: The Genesis controllers work fine as 2 button controllers. I havent been able to test any with autofire. Some of these are getting pretty worn though.
You can get chinese knockoffs for like 4 dollars each. Buy 20 of them as they break quick, and I've had several not work as 2 button inputs.
Master system controller: Only works as one button controller.