Definitely the A93/A939. Not only will you be traveling along Royal Deeside, and through the charming towns of Banchory, Ballater, and Aboyne, but if you have the time, you could visit Crathes Castle, and backtracking a bit, Castle Drum. You can take the Slug Road (A957) from Stonehaven to Crathes, thereby bypassing Aberdeen traffic.
Once you get on the A939, you'll pass through Corgarff (castle worth looking at from the outside), Tomintoul, and Grantown on Spey. It's a lovely drive, particularly the section from Corgarff to Grantown. That's the Old Lecht Road, which takes you past the Lecht ski area, which lost a lot of its popularity once Aviemore and Cairngorm became more widely known (and easier to get to!).
the A96 isn't bad - it passes through or by Inverurie, Huntly, Keith, Elgin, and Forres on the way to Nairn. All of those are attractive towns, but for a scenic drive, you can't go wrong with the A93/A939. However, if you do decide on the A96, Bennachie (near Inverurie) is a rite of passage for Aberdonians. It's about two hours up and back. Also Huntly Castle and/or Elgin Cathedral are well worth a visit, as is the small Falconer Museum in Forres.
Best wishes for a great drive!