You say best, what budget were you looking at? We can get nutso, you want SQ, a pair of IDQs in large sealed boxes, or for some more throaty sound an ID Max, if you really want to shake the neighborhood an Adire Brahma in the trunk in a nice slot ported box, or maybe a DD 9512, or 9917 for the really bold..Myabe a pair of W7 13s or you can settle for ok sound and ok SPL and go for solobraics like I did, although when these kick the bucket I don't think I'll be using them again, They are hit like mothers especially with 3.27ft^3 each in a slot ported box, but even sealed they are pretty dumpy sounding. I'm a bit out of the loop for SQ these days, but I just put an IDQ V2 12 in my wifes car and it's really happy. For more spl subs above what I've already mentioned, look into Shocker Quad Extremes, MMATs juggernaught, Treo Engineering CSX series, theres others, but those are off the top of my head.

The best advice is go out and listen to some yourself, figure out your budget, then go from there.