Depends what you're interested in. Whether it's crumbling old villages, steep gorges, the sea, high mountains, châteaux, solitude, space France has got the lot.
If I was doing your trip, I'd try to include the Pyrenees (you'll be hard pushed to avoid them!),
Carcassonne (rebuilt medieval city),
Mazamet (becasue the road from Carcassonne to Mazamet over the Montagne Noire is great),
Castres,
Albi (fortress-like cathedral),
St Affrique (lovely roads between Albi and St Affrique, plus the Gorges du Tarn),
Millau (the bridge is more spectacular from underneath, but the roads to the south of Millau feel like north Africa in parts),
Séverac-le-Château (campsite, château on the hill, free autoroute taking you past a very sparsely-populated area),
Aurillac, (very wooded area, almost no traffic)
Aubusson,
Argenton-sur-Creuse, Châtellerault, Chinon, Saumur, (lots of châteaux, serene scenery, troglydte dwellings)
Le Mans, (need you ask? The Mulsanne Straight is there for the riding)
Alençon, Argentan, Falaise (hills of
Swiss Normandy),
Bayeux, (lovely old city, as well as the tapestry and war museum)
Carentan (and then stay of the N-road and take the coast road on the east side of the Cherbourg peninsula)
On the other hand, there are loads of alternatives...
Whatever you do, try not to use the autoroute unless you have to. Even avoid the N-roads if possible.
If you're using a Michelin map, any road with a green line running alongside it is considered "scenic".
Most of all, take it easy, follow your nose if a road looks interesting and don't be too rigid in your plans.