druidh – have you done the Carn Ban Mor ride yet?
Anyway, shit ride (IMO) but awesome scenery and great wilderness feel: Skye
Carn Ban Mor (and indeed any riding round Glen Feshie, Coylumbridge and Glenmore)
Any Ochils route (i.e. Dumyat, Stirling)
Devil’s Staircase, Glencoe
I wouldn’t worry about anywhere being busy in the north of Scotland – its not condensed like the Lakes and Peaks are.
Another book I think is worth getting is this:
http://www.amazon.co.uk/Bike-Scotland-Trails-Guide-Mountains/dp/0955082226