I always seem to gravitate to mahogany back and sides. I have guitars with Sitka, Adi, and Mahogany tops with hog b/s. Something about the warm, dry fundamental tone that I love.
Based on the handful of different tonewoods I played, I have to say my D46 is my favorite sounding acoustic, so the spruce top on solid ash back and sides (and neck even) is my vote. To my ears it makes for a crisp clear articulate balanced tone, and it just cuts through. After that, spruce on hog all day! Flat back, arched, doesn't matter