I also should have referenced the PC Engine gamepad too as that of course came out even earlier than the Mega Drive. Its d-pad was also a whole load better than the feeble d-pad of the SNES pad.
Given the frequency with which SNES/Nintendo fans love to remind us all that it was Nintendo who reputedly first designed what we now know of as the d-pad back from the Game & Watch days (as opposed to four separate buttons for up/down/left/right) one would have perhaps thought they'd have come up with a decent one for the SNES pad...
Back on topic... it'd be interesting to hear of what other USB gamepads suitable for the OP's purpose are any good. The Saturn USB pad would be a no-no imho due to the difference in button layout... even though it's obviously a great overall design. I'm not sure what else is available to buy but, along with the OP, would be interested to learn of what's any good.
Despite what I've said here I do in fact own a SNES pad. I bought it for use with SNES games via Wii's VC for a more 'authentic' feel. I bought the SNES pad-to-GameCube/Wii adaptor as sold by that company referenced earlier in the thread. It certainly feels authentic - authentically crap, that is!

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