If Nintendo had to, which one?

[Soundwave]

Nintendo wouldn't align with Disney or Apple or Microsoft IMO because they're all massive American corporations that could then buy Nintendo out and do whatever they wanted with Nintendo IP and their staff.

Sony isn't big enough to be able to buy Nintendo and they are Japanese. That's probably the direction they would choose overwhelmingly. They could more or less just continue to function as they are now allied with Sony.

Look at their smartphone games even, they could have partnered with any larger US/Euro smartphone maker, but instead they choose to go with DeNA, a smaller Japanese company.