Microwaving destroys a lot of the nutrients in food. Sweet potatoes steam very nicely if you cut them in slices, and that does preserve the nutrients the best.

The reason for boiling regular potatoes is that boiling washes some of the starch out of them, so they're fluffier. There's no real need to boil anything else - except nettle tops maybe, but I don't guess you go gathering nettles much in Brooklyn. :)