Inverness, or Fort William
because there are still some great roads South of the Great Glen as well as North of it. You still have access to cheap(ish) fuel before you head off, with a spider's web of routes emanating from either city. While Ullapool is also a good choice, you still have to ride the same roads for quite some distance before anything different gives you any choice for the day.
Inverness or Fort William gives you access, within a day's ride, to any of the following:
Ullapool, Poolewe, Applecross, Lairg, Durness, Kylesku, Kyle of Lochalsh, Skye, Mallaig, Fort William, Glen Coe, Tyndrum, Crianlarich, Killin, Aviemore, Tomintoul, The Lecht, Braemar, Glenshee, Blairgowrie, Oban, Inveraray and many others.
These are the roads that car and bike magazines take to for dramatic scenery and therefore photography, little traffic, an ability to push it a bit without being accosted by the Old Bill, a great variety of straights and corners, and of course great food to suit any budget, from a burger van to Michelin starred restaurants.
For bikers there is, of course, the added bonus of Wild Camping. No need to share campsites with caravanners and the like.
Oh, and no North Wales police!