In reply to Ben_Climber:
Sanna's lovely.
I know of a number of beaches even just on the mainland that I think are as good, or even better, but I'm not going to list them online! And of course, the Outer (and some of the Inner) Hebrides offer an incredible wealth of mindblowingly-beautiful beaches.
I will say that I find looking at maps of the west and north coast, finding sandy beaches marked, and going exploring, to be a very rewarding activity.
Without giving away the best beaches, I can suggest a couple of worthwhile trips if you're looking for more remote ones where the effort is part of the reward.
Have you ever been to Peanmeanach? It's a 5km walk on a very good path on a semi-remote peninsula, looking out at the sound of Arisaig. There's a very pleasant part-sandy beach, and views across to Rois-Bheinn and the other Moidart hills; a very different situation from Sanna but still gorgeous. There's a bothy, and plenty of space for camping.
A bit further West, and the beaches at Port nam Murrach and Port a'Bhathaich are worth a walk, and take you a little bit away from the hordes at the (admittedly stunning) beaches on the strip north of there.
ANd I don't think it's giving away anything too much to mention Cuaig beach, on the Applecross Peninsula, as a totally different sort of beautiful beach - a short walk from the road, this time with redder sand formed from the sandstone rather than shells. And there are small cliffs north of it suitable for a spot of bouldering or short soloing; one time I was there, an otter scurried off through the boulders below them.