I was just about to paste in the 'neeps' link when I saw I was beaten to it..
that is probably the best reference for the 'good' Aberdeenshire mtb routes - with maps and descriptions
my favourites are the Glen tanar routes, e.g. fungle/firmounth,
Also the one near GlenFiddich
and
the Loch Muick circuit is a classic - one of great UK mountain rides,
Pitfichie and bennachie have good singletrack - as does fungle,
there is a good route from Braemar, up Glen callater, then back over to Glenshee road, then up and over Morven (naughty)
there are also good rides up Don valley - e.g. good singletrack by Loch Builg near Ben Avon
other man made trails include Keith/Ben Aigan - the ben agian trail is looking and riding well
these are built and maintained by the moray mountain bike club:
[
www.moraymountainbikeclub.co.uk]
further afield - anywhere around Aviemore or Laggan Wolftrax
andy