In reply to Mr. Plod:
Further to all of the above, if you're camping and can be flexible, if you make several alternative plans and simply choose to go where the weather's best, you'll have the best chance of having a great time.
If the weather's pushing in from the east, head to the west, if it's pusing in from the southwest head to the northeast, if it's blowing down from the north you may be best in the Southern Highlands ... you get the idea!
It's a hilly enough place that you can generally find decent, or at least better, weather in one corner or another.
And personally, if it's good everywhere, I'd head to the North West (Kinlochewe, Gairloch, Ullapool, Lochinver or Durness), or to Skye.
Post edited at 17:10