There are a few brands currently which offer direct plug and play; Match, Ground Zero, Eton, Focal to name a few.
Depends on your requirement really, if you are just looking for upgrade over stock speaker, then upgrading to a good quality front speaker+tweeter will suffice. If still not satisfied then upgrade the underseat woofer as well. Another step forward will be as rssid mentioned, to add a DSP amp or amp.
I had the stock speakers previously. I bought my own tweeter cover online and installed the Ground Zero front speakers+tweeter. For me the sound quality is much better than stock. Next step for me is to upgrade the underseat woofer + DSP amp.
