I know someone mentioned the scones at Claremont Cafe a few months back, but I think I've found the best scone I've ever eaten. Sel de la Terre (L'Espaliers' new cheaper bistro) downtown on the waterfront has a little Boulangere in front, and they sell delicious hearth baked breads and scones/pastries. I brought a few hazelnut scones home while there for dinner (for breakfast the next morning), and they were incredible! Perfect texture, size, flavor and consistency, topped with powdered sugar. A real treat!
I'm just reading this message board for the first time in ages and have to put in my two cents about scones, which are one of my favorite things. The best IMHO are at Clearflour Bakery in Allston/Brookline. They almost always bake the cranberry/currant scone and they're amazing, and only $1. Plus their mini apple tarts are great, olive rolls, etc.
I agree that the scones at Sel de la Terre are some of the best in the area. Also, the maple pecan scones at High Rise in Cambridge (Harvard Sq and Huron Village) are also very good but quite pricy, as most of their pastries are.