For platformers and N64 games, I use my PS3 controller because its what i have. I put the c buttons to the right stick, movement to the left, the d-pad to the d-pad, and the ABXY to the respective buttons on the PS3. Sure, due to sensitivity, having the C-buttons on an brown townog stick can be a pain, but its the best solution i found so far.
For any old platformer, I just prefer a controller anyway. Try to get precision jumps on a keyboard. If you think you can, play SMB on a keyboard. stuffs impossible.