All depends how you are getting to Fort William in the first place and then you can have fun stitching together your overnight locations.
From the south, go west as early as possible and enjoy any of the roads from Crinan up through oban and up the west to Fort William.
From Fort William, all the west coast stuff mentioned is great, you really are spoiled for choice. Fort William to Inverness, the A82 is the least interesting route, the A 86 and then into spey side has less traffic but all the west options will be far more fun.
From Inverness to Aberdeen, at all costs, avoid the A96. A boring tractor infested, inadequate route joining two major cities.
Instead, go to Forres, head shouth across the moor to Granton on Spey and then take the A939 cockbridge to tomintoul route across the cairngorms. Through the lecht ski area and then down into donside.Head for Kildrummie castle and then alford, but the exact route doesn't really matter, as you can go a bit further south and cut across through Deeside instead.
Only warning: At this time of the year there is a lot of potato seeding, ploughing, harrowing and general sowing/fertilising going on so this means a lot more tractors on the roads than would normally be the case, particularly once you get into Aberdeenshire. Lots of big, wide heavy implements going slowly in odd places.