“Natural” tracks – I don't really see the point in discussion. Limiting it to tracks designed for bikes that I've "done" (bear in mind I'm a proper mincer and haven't ridden any designed tracks south of Hamsterley) Laggan Black is probably hardest XC route. DH track wise, Glencoe (only admired from above) and Dunkeld tracks are pretty stupid, Kidland is a bit mental, Fort William is hard and its difficult for its length alone, and I can't even get down all the stuff at Innerleithen cleanly so I'm not the best judge. That youtube clip makes Aberfeldy look hilarious though, makes me want to try a full sus.
One for the 'potential mountain rescue' type of hard is supposedly Cross Fell. I really want to have a crack at this if we get a spell of decent dry weather this summer!
I think someone has been pulling your leg. Not remotely difficult riding (though nice), have been riding the tracks up there since a kid without a second thought, usually take the cx bike nowadays. Obviously weather can be bad and there aren't many folk about so you need to be able to take care of yourself but same can be said of loads of areas with decent riding.
Locahan Fada down past Slioch must be the place for over the bars crashes. I had a big stack there – luckily it was reasonably far down. Hurt like hell and made me think a bit – mainly that I should have had my shirt on (in my defence it was 30 degrees), and that if I hadn't been lucky that I had seen 3 other people in the last 2 days. Great area for riding though in a less remote spot with less consequences if it goes wrong, more than a 100mm fork or even full sus I'm sure the same track wouldn't be quite as challenging. I certainly wouldn't have it near the “hardest” track in the country or even the hardest track in a 5 mile radius – harder than me though.