I don't think there is any one particular "must see" in the highlands. There's plenty of natural beauty and a variety of things to see and do.
You say you are going to be "based out of Edinburgh" -- does that mean you will sleep in Edinburgh each night and use the rental car to take a day trip each day? If so, I would rethink that and sleep somewhere in the highlands. For example, if you really want to see Loch Ness, book two nights in Inverness or nearby. That will give you (day 1) time on the drive up from Edinburgh to stop and see things that interest you, (day 2) ample time to see Loch Ness with Urquhart Castle and the nearby Loch Ness Centre & Exhibition, and (day 3) time on the drive back to Edinburgh to stop and see other things you didn't have time for on the drive up.
If you are committed to sleeping in Edinburgh each night, then you'll want to take shorter day trips in your car, perhaps in three different directions. For example, you could go to Pitlochry & Blair Atholl one day, the Trossachs another, and Stirling, Falkirk, and Linlithgow the third -- or use the third day to go out to the coast either by North Berwick or north of the firth to Anstruther.