Best for what? It's a very relative term.
If you want upgraded sound with bass on a budget, you can buy a nice $100 cd player head unit from Pioneer, get drop-in Pioneer 6x8's all around for $100-120 for two pairs, add a Pioneer sub amp for another $100, and a sub box with 2 12's for around $200.
So about $500 for good quality gear and a setup you can crank to the max and still hear the vocals and get ground pounding bass.
Alternatively, if you just want to vibrate other vehicles, you can get some cheap sub/box combo with an amp for a few hundred, hook it up to the factory system, and drive around with the bass vibrating all the loose sheet metal on your ride so the drivers of all those cars that your bass is shaking laugh at you. Bandpass boxes in particular are great for shaking everything if you just keep playing the same track on the bass CD that has it's resonant frequency. This setup is also fantastic for attracting the attention of the local authorities and contributing to your local economy by receiving numerous citations for noise ordinance violations.
If you want really high end stuff, get seperates and kick panel mounts for the front speakers or use the stock door location for the midrange and get sail panel mounts for the tweeters, get some matching components with an adapter for rear fill in the stock rear speaker location, add matching 12" subs in the rear in a custom made box with an amp rack to hold the amps for the mids/highs and subs. It'll end up over a grand at least, maybe 2 or 3, but it'll sound great, the people a few blocks away will think there's a mobile concert coming.