The best stock system I've ever heard was a 2006 BMW 760i. Still was nothing special, even though it was a "premium 17 speaker system" or whatever ridiculous amount of speakers they used in it. I have an '01 Denali with the "premium Bose", and at moderate listening volumes, it sounds pretty good, once it gets turned up though, it turns into all mid bass and midrange, the tweets seem to disappear. Pretty good mid bass though, deadening the doors, changing the locations of the tweets, and adding a good sub would make for a pretty decent system for 98% of the population. I plan to add a sub and call it good for now.
On the flip side, I've heard "$1000 custom systems" in friends cars that sound worse than my Denali does stock. If car manufacturers, and people putting in custom stereos, worried more about install and quality than the number of speakers, we would all be better off.
A good 5 speaker system is all people really need: 2 tweets, 2 mid basses, and a sub... Or a good 7 speaker: 2 mid basses, 2 mids, 2 tweets, and a sub... Quality>quantity, 20 crappy speakers is far inferior to a properly amp'd set of comps and sub.
I'd really appreciate a "factory stereo delete" rebate, that ships the car without a HU, amp, speakers, etc... And gave us the option to outfit the stereo ourselves. If I could save $500, I would. When I buy a car that I plan on keeping for the long haul, the first thing I'll do it strip out the stock stereo and do it my way anyways.