What I've learned in car audio is that certain brands are good for certain things while they suck in other areas. So here's a guide for putting an audio system in your car:
For the head unit I would go with- Alpine, Nakamichi, Eclipse, or higher model Pioneers. These 4 brands offer the best sound quality over any other brand.
For subwoofers stay away from name brand stuff. JL Audio, is good don't get me wrong, but they're stuff is way overpriced. Check out
www.edesignaudio.com, www.reaudio.com, www.ddaudio.com. These are just some of the subwoofer companies that a lot of car audio competitors use in dB drag racing. I bought my a-series 12" woofer off of edesignaudio.com and it's the best sounding subwoofer I've heard. It compares/beats the JL W7 in my book, and costs a lot less.
For speakers stay away from sony, pioneer, rockford fosgate, etc. They're speakers may sound decent to you when you buy them, but it's really not possible for them to sound awesome. I'd go with CDT Audio, Dynaudio, Image Dynamics, Focal, or build a set from
www.partsexpress.com and have
www.madisonsound.com build the crossovers. Also get a component set, they offer much more detail then just replacing the stock speaker in your car.
Amplifiers, I have no clue, I'm still researching on this subject, from what I know so far go with PPI, Directed, Viper,
www.ddaudio.com, Avionixx, Tsunami, and stay away from name brands. Again JL Audio is good but they're **** is overpriced.
Hope this helps.
what are good speakers 
