True... newer cars have better and better audio systems in them and are more than enough for most people... but for those seeking more.... Dynaudio, Focal, Morel and Diamond Audio make very, very good sounding car audio drivers and crossovers.
I am using the Dynaudio 3-way Esotec system with 7" midbass unit (actually 6.5" I think, but they call it a 7"). I did all the install and had custom thick MDF baffles CNC cut for the mid/tweeter pod in the kick panel area. I installed the midbass in the stock door location (with cutting and more custom MDF rings). Also used almost 100 sq. ft. of Dynamat Extreme (necessary to cut down on road noise).
I am running three Diamond Audio monoblock amps... one to each component set at about 240 watts per side and another monoblock running about 400 watts to a JL Audio 10w7 (sealed). I love the JL subs (either w6 or w7 series), I have owned a few... the 8w7 is totally badass... had it in a slot port box to spec. I am using an Alpine head unit, always have. The system sounds amazing. I have sensitive ears to high frequency and have owned some serious component speakers (Boston Z6 and Diamond Audio Hex Series) and to me, these are the best sounding car audio speakers for the money. The big dome midrange is awesome!!! Also, I used Audioquest cabling throughout. Setup (crossover points and EQ) are vital to get the right sound. These decks now have a lot of flexibility and settings to cutomize the sound. I think I paid around $1,100 OTD for the Dynaudio components from a dealer.
Took maybe 60 hours to do the entire install. And this is setup for SQ (Sound Quality), not SPL. When I play Gotye "State of the Art" or any Gordon Goodwin people freak out at the sound.