Stainless, great but not exactly cost effective because the conditions they're designed for would destroy your guitar. Titanium, WAY too expensive, and I've never seen Ti wood screws. Solid brass screws are really too soft for general use and break too easily, I generally avoid them. Aluminum, never seen Al wood screws, can't imagine they'd be very strong. Plated steel, (chrome, brass, galvanized, cadmium, nickle, etc...), strong and inexpensive. (When I mention strength, I mean resistance to breaking off during initial insertion.)
I use chrome or black plated steel. But as they say, whatever floats your boat....