Easy choice for me, Nintendo.
The reason I suppose is that I like their own franchises, especially Zelda but also the Mario games. And now and then it pops up some new interesting games for their consoles. A lot of nostalgia for me since Nintendo created console gaming, for me at least. Despite the lack of third party support their consoles are haunted with, their own games always deliver.
If I had to pick a secondary it would also be pretty easy, and it's PlayStation.
The reason here is pretty much the same but with less nostalgia to it. They have some game series like Uncharted that I really enjoy, and they always have "the better" console. Be it the very best or one of the best, I can always rely on the hardware. Many in-house devs and good third party support as well.
As for Sega and Microsoft I have never really felt anything for them. I did enjoy the Sonic games however, and now that MS has the Banjo series I can only grow to like them more. I hope they can really make a great new installment with Banjo Threeie, I wish for it to blow everyones minds and just be as awesome as the other two games. Oh how I miss those games...