Strangely enough, the Xbox Series X. I've had a PS3, PS4, PSP and Vita and only ever owned an Xbox360 which I later sold and replaced with a PS3. But this time I like Microsoft's approach. They have the more powerful platform, invest more in exclusives than before and generally seem to avoid mistakes (unlike last generation). Also, I like the controller layout better but ultimately still bought Playstation products.
The backwards compatibility is also great and is going to become ever more important, as we can see with the Switch and with more and more actual classics available. 20 years ago there was only a handful of classic games (as in high quality, timeless masterpieces) as gaming was still so young. But the number keeps increasing each year and Microsoft seems to be going all in on this one which I like.
I also expect the Series X to be more successful than the Xbox One and the PS5 to have somewhat lower sales than the PS4.