Probably Sony, because they have given me the games I enjoy most. I only own a PS3 this gen (home consoles. I have a DS, PSP, PC), and I have to say it has satisfied my needs.
Some of the Wii's games interest me, though not enough and not as many as to justify a purchase. If Nintendo's next console ends up being appealing to me I may get that too down the line.
MS would have to change their offer a lot for me to be buy their next console, because nothing they have interests me at this point. I don't see what MS has that no-one else can offer, bar Kinect, Live and exclusives like Halo, Gears and Forza, none of which I'm interested in.
In any case I'm open minded and anything can change next generation.