I'd go with Kander and Ebb. I know this is biased by my age and taste (maybe if I was a bit older and cared more for the golden age versus the darker and rougher edges of K and E, R and H would be the obvious choice).
I also wonder about one hit wonders. Naturally we think of those that have a greater volume of output. I think in this category I'd go with Tony Kusher for the lyrics to Caroline, Or Change. It's a breathtaking achievement, worthy of the perfection associated with Sondheim. I especially like how he uses some of the colloqualisms to produce perfect rhymes that surprise since they aren't always grammatically correct, but make sense for the characters in that time and place.