Bread

Lowest calorie per weight -- the one with the highest water content.
Lowest calorie per what your body will digest -- the coarsest one with a lot of kibbled and whole grains in it, and lots of bran (fiber).
Lowest calorie per slice -- the lightest, fluffiest one. That toasting bread at Vietnamese bakeries is sometimes 30g a slice.

Seriously. I bake my own bread so I know what I am talking about. In bread baking, you can at best put flour:water 1:1, but only whole rye will hold that up and make a decent loaf that way. And it will be dense, so not low-calorie per slice, but one slice will last you much longer. And consider that in the baking process, some of the water evaporates.

Now consider that any grain that is able to form a bread is quite pure starch (carbs) and some protein (up to 12% or thereabouts, usually 10- at most 15), and any flour milled out of grain is relatively dry, so it hovers just under the 400 calories per 100g mark. The more fiber is in it, the lower, so you get 360, 370 calories for a good whole meal. If you mix that 1:1 with water and bake it, you get at best something just over 200 calories per 100g. Or 100 calories per slice (assuming the slice is 50g).

That's ideal. Wheat can't hold as much water as rye; in wheat breads, the flour:water ratio can be as bad as 3:2. And some bread have added oils. So a white bread can get up to 260cal/100g.

If you are looking for some really good, if dense, wholemeal rye breads -- don't fall for what they call 'light rye' nowadays in some bakeries. That contains in the vicinity of 70% white plain (wheat) flour. A rye bread made with the 1:1 water:flour ratio can't be shaped into a round loaf, it has to be baked in a tin. If you are in Melbourne, travel to Balaclava and look for Baker In The Rye in Carlisle Street, they have some lovely Eastern European breads. Again, I can't guarantee that their breads are extremely low in calories per slice, but relatively to their satiety value, they are. They are made with sourdough (no other way to leaven rye properly).