The West Highland way up to Inversnaid is a good ride, getting progressively more technical as you go. After Inversnaid its a nightmare though, so ideally you would either double back or get the ferry across the loch.
Arrochar has a couple of good loops, a bit forest road heavy in places but still worth a ride. One goes around the Ardgarten Peninsula to Lochgoilhead, up to the waterfalls above town, then back round to Arrochar on the forest roads above the Rest and Be Thankful.
The other heads up to Coire Grogain via Succoth, and descends the singletrack beneath the pylons, which is pretty steep and rocky in places.
Both the Arrochar loops are in Kenny Wilson’s ‘Mountainbike Highlands’ book.
Good loop from Auchindrain nr Inveraray too, over the Leaccan Muir, however I think a lot of the good bits have been flattened by the forestry recently. Still worth doing once at least even if they have.