I have learned that to my ears, mahogany back/sides/neck and spruce top are the winning combination. To me, it's what a well-recorded bluegrass guitar always sounds like. I don't play bluegrass myself, don't have the chops, but that tone, especially from single-note melodies, is so earthy-sounding that I've just got to have it. Because I'm used to the pronounced midrange of mahogany, I have always found acoustics with rosewood backs and sides to be hollow-sounding. To my ears it sounds cheap. Which is crazy, I know, because so many people love that sound and will pay a premium. But give me mahogany every time.
A more interesting, lesser-used combination is the cherry back/sides and cedar top that you'll find on some lower-priced guitars. I have a Seagull S12+ that is probably the best 12-string I've ever heard. And at a sub-$500 price point, that's really saying something.