There's no "best" speaker out there. It all depends on what fits your budget, your wallet, your skill level, and (most importantly) your ears.
Drop-in coaxials (5x7/6x8 sizes) are the easiest and cheapest. They'll make a definate improvement in sound.
A bit of an upgrade over plain 'ole coaxials would be plate components or coaxials with a separate crossover. They'll still drop right in (5x7/6x8 sizes). The crossover is the key and improves accuracy quite a bit.
A step up from there would be a set of components with separate drivers. The woofer is usually larger than plates'/coaxials' drivers with more power handling and a longer throw (X-Max). It'll be able to produce better mid-bass and go lower in the frequency range. Tweeters tend to be made of better materials and can also handle more power. Components take more work in mounting them. You'll probably have to make a baffle for the woofer and decide where to mount the tweet. You could also use kick panels to improve imaging. You could go with as big as an 8" woofer in the door. You could also go with a 3-way component set with a woofer/mid/tweet. But, as you make the step up it's $$$.
Almost forgot, you could adapt some 6x9s in your door. But, as you're going to run a separate sub it'd be a waste of time/money.
If you could come up with some $$$ figures and how much of a project you wanted to tackle, there are many on this board who could throw out some suggestions.
