Really depends on what kind of brand you like. As Red GTi, she seems to know the best, at least I think she does.
I prefer (I'm only 16) MTX, Alpine, and JL Audio. Now there are a plethera of differnt brands out there, I only reccomend these three because I have used them. Look at my sig you'll see what I a using in the way of Subs. MB Quarts are very good, I've heard nothing but good things about them.
Search the setups in the Show off your ride forum. Several awesome setups there.