Honestly, I am not sure I have ever heard a great factory sound system in a car. In most cases, even some cheap aftermarket $50 speakers are an improvement. However, most new cars are satisfactory for at least the first couple of years. Sadly they use cheap paper speakers that sound decent to sometimes really good for the first couple of years until they start experiencing minor deterioration. In my Protege, I have a Kenwood Excelon HU, tweeters in the front with 2 Kenwood Excelon 6x8s, kenwood Excelon 6x9s in the rear, and 2 Kenwood 10" subs. I wouldn't say it is audiophile like a really expensive Focal system or JL high end setup would be, but it does sound great and can handle some volume before distroting. You can hear every note if you tune it to be that way. I listen to a little bit of everything from Classical to Rock to metal to POP to hip hop. The system is a great all rounder, but could be better for SQ if that is your goal. My 85 RX7 has a Sony XB100BT with a 40 watt RMS AMP, 2 4 inch Kicker CS speakers, and 2 6.5 coax Kicker CS speakers. This system sounds a bit more audiophile style and tends to be less used as I can barely hear it on the freeway due to horrible exhaust drone. It could probably benefit from a small 8 or 10 inch sub and sound even better.
My little brother has a 2016 Mazda3 that doesn't sound great or bad. It is better than passable, but you can tell the audio system is set up for what they think the average person wants to hear. It doesn't go down terrible low, but it does sound really boomy for the 80 hz range that is common in most Pop style music and would actually be a pretty enjoyable experience with the addition of a small subwoofer. I just bought another protege that had the stock speakers. The girlfriend was somewhat satisfied with them as it was so much more compact than her grand prix and it did sound oddly better. I replaced them with a set of Rockford 6x8s and 6x9s and the first words that came out of her mouth were WOW. There are some audio systems that come out really good from the factory such as special Kenwood, Pioneer, Sony, Monsoon, Infinity, etc that I wouldn't bother changing. Personally, I am not a fan of most Bose factory sound systems as they sound more catered to listening to a movie than listening to music.