Ok, well then I'm going to assume you have a car with no space constraints and an unlimted budget.

I'd do 2-3 high quality 12's in a carefully designed and constructed ported enclosure with an absolute ton of power so as to have a ton of headroom for maximum sound quality.
To support this power I'd upgrade the alternator as large as possible, and possibly even add a second one if the vehicle allows. You can never have too much charging system. I'd upgrade all stock wiring and toss the stock battery and swap in a nice new one, and add a second one in the rear for extended playing time and current capacity.
I'd buy the best sounding amplifiers I could get and power the hell out of a high end set of components up front built into custom kick panels if this theoretical vehicle allows for it. Actually I would use 2 sets of these components because volume is obviously a desire here, and we want to make sure it's there. In the rear I'd do a single matching set of these components, also with lots of excellent power.
Up front I'd pick a source unit with a good DAC and whatever features i want, and run it into a good EQ for maximum tuning purposes.
I'd sound deaden the entire car and eliminate as many rattles as possible.

And I'd have an awesome sounding car that gets ridiculously loud. I might have spent 10,000$ or more, but it will jam. Now of course you don't have to go anywhere near that extent to have good sound, but without a little more clarity I'm not sure anyone can really help you.