I've heard good things about the iBuffalo SNES controller and the 8BitDo SNES controller, which of these is recommended, or is there another controller that's better?

really no. they are both insanely good and well received.

The only thing I have to say, is be careful with the Buffalo controllers. Depending on what u are trying to do they may give u some complications. I run a RPG game that requires a lot of diagonal inputs, and my friend has one of the buffalo controllers but it doesn't pick up diagonal inputs which makes it impossible to play Not impossible but very frustrating to speedrun

SO if it is JUST for SMB1 u should be totally fine, just be aware for some reason his controller was having a hard time mapping directionals Just wasn't doing it and I have heard of other people having this issue as well. Good controller other than that, I would probably just get an adapter to use a liscensed SNES controller tho personally

The 8Bitdo one is the best by far but, you will need to buy an adapter for your PC if you have chosen to buy the 8Bitdo one. Buffalo sucks because of the no diagonal thing. You could also try to find USB dogbones :D Or, what i would do: Getting an original SNES controller and an adapter for my PC :)))))))))))))))))))))))))))

Xbox One Controllers actually work pretty well, as RoyLGamer uses it. I also use it when doing emulator and it works fine for SMB

I used a Xbox 1 controller to run for awhile before getting a cap, but I have to say they seem to break kinda easy after intense use. Like I played a lot for maybe 4 months and the d pad is pretty broken drops inputs and the b button is smashed in from mashing. Could probably fix this easily, but would be cheaper to use a better controller imo, also very different feel... at that point I might as well be a nerd and use a keyboard for optimal inputs ~

I don't think different controllers are better than others (unless there are restrictions like Xein mentioned about Buffalo controllers not allowing diagonal inputs), I think you can use whatever you want I see no disadvantages between a dogbone or a keyboard. It is important to get something that fulfills all the mechanical needs.