Tywi track in Mid Wales-
or Google 'Strata Florida landrover'. Wet, fun, fast in places, technical in others and superb views when you reach the top. And a bothy to boot.
Or the Dan yr Ogof Descent. Mentally fast, super riding, coming as the climax of a LONG moorland ride. Few people do it (to their loss) because of the long approach. 35 km of well rewarded hard work and Beacons views. With a lot of getting lost potential too.
the ride from Geysir to Hvitarnes in Iceland. You ride through the middle of nowhere and then turn off the track onto a smaller even sandier and wash-boarded track. And then later, onto a smaller one still, hoping to be greeted by the sight of a mountain hut close to the glacial lake. 35 km of pure slog and satisfaction.
But for pure mtb-fun. The Ardent loop via Morzine- Zore- Col de Joux Verte- Abricotine- Mosette- Lac Verte- Col de Chesery- Les Linderets- Ardent- Lac du Montriond- Morzine. 30 km of pure fun.