Most steel stringed guitars have a spruce top, but spruce requires a fair amount of bracing, at least in guitar work. I would consider mahogany. I use it extensively in ukulele building, especially for tops. When resawn to about .075", it has tremendous resonance, and requires little if any bracing. I use a tuning fork (a small metal device used to tune violins) on various pieces of wood to find the highly resonant ones for my musical instruments. The fork can also help locate loose braces and cracks acoustically, since they buzz.