I think a partnership (not an outright merger) with Sony would be best for a couple of reasons even if some gaming fanboys can't see past them being "rivals" (in a pretty loose sense these days) nowadays.
For one, Sony is not big enough to force Nintendo to do things they wouldn't want to do. The problem with joining with an Apple or MS or even Disney, is those companies are large enough to purchase a majority stake of Nintendo shares, and could then run the company into the ground (see: what MS did to Rare). Part of me thinks Apple would cultivate and allow Nintendo the time/resources to make quality games, but it just as easily could turn out that they want them to pump out a ton of 99 cent Mario/Pokemon apps with a ton of DLC.
Sony doesn't have that level of cash, they'd have to work together with Nintendo.
Secondly Sony's president is Kaz Hirai, a Japanese video game veteran ... so he would understand and know how to talk shop with Nintendo's management ... Apple/Disney/MS' head management are not from the game industry and just view the business as another branch. So if Nintendo has a problem they can go straight to the top and talk to head honcho, and he will know where they're coming from.
Capcom/Sega ... hate to say it but both these studios are past their prime and have lost most of their best employees. Nintendo needs fresher IP too ... the most "recent" hit franchise either of these studios has come up with is Monster Hunter, and Nintendo already pretty much has exclusivity over that.
Capcom would probably let Nintendo make Megaman games already if they really wanted to, so that just leaves Resident Evil, which has seen better days, and Street Fighter, neither of which really dramatically change Nintendo's position in the marketplace.
