I don't think there's a single best time - even winter's good if you like winter sports.
Aberdeenshire can get very windy, particularly cold when it's from the east - in spring warm weather can bring "har" a cold sea mist along the coast - but it's a great area for beaches, cliffs and wildlife, and not very touristy.
Inverness can get busy, particularly anywhere near the North Coast 500 (NC500) tourist route - the Moray Firth is the sunniest part of Scotland as it's in the rain shadow of the Grampians for wildlife, visit Chanonry Point a couple of hours before high tide as a pod of dolphins feed on the salmon and generally play around, Spey Bay where Osprey can be watched while they fish and there's rumour of a large monster in Loch Ness!