There is no real answer to this, as none of them are essential. Vacuums get filled.
As for those saying that the first-party games of any of them "couldn't be replicated", that's just silly. Games are made by people. These people wouldn't just suddenly die if x company dissolved. They'd simply go elsewhere, maybe start their own company.
I do get the feeling, however, that some people wouldn't "like" those new games as much if they didn't have a certain company logo on the cover, which is just kind of sad.
However, if I had to say which was most important right now, at this very moment, I'd pick Sony by a slight margin. Nintendo is too backwards and self-absorbed, while Microsoft feels more like they're using gaming as a means to an end. Sony, to me, feels like they're going in the right direction in terms of hardware and third-party and customer relationships.
