The way things currently sit, I guess it is Nintendo.
People saying Microsoft leaving would be best don't know what they're talking about. Games library is the least important element of this debate, as any library would go multiplatform the instant one of these companies went under.
Whilst Nintendo has offered great and unique innovation, the reality is thats only occurring every second generation these days, and it just isn't keeping them as competitive as the other two players in the game.
Microsoft has been innovating consistently and effectively. Sony has embarked on a long process of trying to keep up with ideas Microsoft has already had, implemented and made mainstream (on-line, controller-free motion, cross party chat, multimedia powerhouses etc). Sony's idea of hardware and services innovation is just more power. Whilst all three companies are innovating within games, this is not relevant to this specific question as, again, IPs and studios would go multi-platform immediately. Nintendo has been the genesis for many great ideas (like motion), but it has consistently been Microsoft that has found a way to make them mainstream, expand them etc.
starcraft - Playing Games = FUN, Talking about Games = SERIOUS