For the least amount of remapping for MAME, especially 6 button fighters, I'd think you'd want
1237
4568
If NEOGEO or Vectrex is your main thing, and you want to use the top row, you could use
1234
5678
Overall, I don't think many people are going to expect the primary row to be the lower one. Of course, you can remap to your heart's content in MAME, so it's really the other emulators that you need to think about.