I've ran JVC,Kenwood,Jensen,Audiovox,Panasonic,Clarion,Blau punkt,Alpine,Sony, and Pioneer just to name a few..
The cleanest sounding head unit was the Clarion..It was one of their higher end units..
As far as reliability goes..The JVC,Kenwood,Jensen,Audiovox,Panasonic,Blaupunkt, and Alpine didn't last very long..
The Sony and Pioneer were the most durable, and did not die..
Pioneer makes some really nice head units, if you don't mind spending a few hundred on one of their higher end units..
I don't mean feature wise, I mean sound wise..The higher end Pioneer head units sound a lot more crisp..
Just about all of the head units you can buy at places like Walmart, are just entry level units..They don't sound very good at all..
I'm running an older Sony head unit in my Thunderbird now, that has all the bells and whistles..It sounds pretty decent too..
My suggestion, if you're not happy with the way the factory stereo sounds..Is to go with either a Pioneer or Sony head unit..
Don't cheap out either , if you care about sound quality...You get what you pay for..
Even with a nice set of speakers, a cheap head unit is still going to sound like crap..