Anywhere but here 😉
For ease of riding, but not necessarily interest, the Netherlands is great, as is Belgium in my experience. Sustrans style stuff can be had across most of France. For actual XC mountain biking I’d have the Jura high on my list (have been, but not ridden there, but heard good things). I have ridden off-road and dirt roads in Tuscany a bit which I think has quite a lot of potential.
Alternatively I know a few people who’ve cycled the Camino de Santiago in Spain and loved it – long distance, dirt tracks and cycle routes, etc.