I am in the process of installing some aftermarket stereo equipment in my Sonata. I am taking many aspects in play on this install such as keeping the stock head unit for less theft chance but am adding all new speakers and an amp. A sub will come in the form of a single 10" ported as I am looking for more SQL than SPL.
When opening up this question you will ultimately need to ask yourself what you are really looking for by way of sound. You will find many different opinions on this and most will not be much help unless you have a goal you are looking towards.
The stock 6.5" speakers in my GLS look like this:
as you can see the magnet is really small and the construction isn't that nice so you will not be getting much out of this type of speaker.
Things you should focus on are:
What type of sound am I looking for? Headbanger Loud, Symphony Clean, or Boom from a mile away.
What is my budget?
Factory replacement or customized installation? A clean swap or cutting and fitting.
Once you have those in mind you can start to get a feel for what type of equipment you can use or need for your install.
It will help with understanding a lot of the opinions you get back as replies.