I would hate to see any of them merge, competition is great for gamers.
BUT, The one that'd make most sense (realistically) would be Nintendo + someone else.
Sony and Microsoft are huge companies with many many many divisions outside of gaming. If they ever came to situation where a merger would be likely, they'd probably close the divisions down instead.
Nintendo on the other hand are a purely gaming company, which means that if their gaming goes belly up one day, they have nothing to fall back on, meaning they'd either go into the dust (hi atari) or be absorbed by someone else.







