Though the beaches of Harris (especially Luskentyre), North Uist and Berneray run it close, my favourite beach is still this one, Traigh Eais, on the northwest point of Barra:
It's a huge beach, and I've never seen anyone else on it. In sunny weather, it's particularly spectacular to walk along. Here's a picture I took from the top of the small hill at Eoligarry that overlooks the beach:
Just behind here is the croft run by Angus MacNeill, our local MP. He's a cool dude. When not in Westminster harrying the government (it was he who discovered the obscure act of Parliament that says it's illegal to flog off honours for a quick buck), he's back on Barra shearing sheep and doing other crofty-things.
The top of this hill is a great place to sit. From here you can see many beaches on Barra, Eriskay and South Uist; here's the view looking roughly northwest:
Plus, you can watch the planes land and take off at Barra airport on the next beach over to the one pictured.
All the pictures on this blog entry by me.