Land access in Scotland means there can only be one winner.
Ignoring that fact, ISTM that Scotland offers a landscape that is unique in the British Isles, so there are rides there that you couldn't really do anywhere else. Long, wilderness jaunts through pointy mountains. While a lot of these classic Scottish routes are essentially pony-trekking, rather than mountain biking as such, the possibilities for technically challenging stuff out in the wilds are endless if you know what you're doing (see Sanny's long Munro thread on here).
As awesome as Scottish MTBing is, it's still a whole country you're talking about. Stuff is spread out and there are plenty of places to live there that would suck for riding. Maybe there's a spot in Wales that is unmatched for density of high quality trails - a bit like the Lake District offers you so much in every direction in a small-ish area.