Buying from the forums > Sonic/carstereo/woofersetc >
crutchfield > best buy.
Buying from the forums is the best way to go, IMO. You'll get the best deals, on the best equipment for the most part. I've only been into any of this for a year, and it's been the easiest thing to do IMO.
Sonic, Car stereo, and woofersetc are the preferred place to shop for stuff if you insist upon buying new.
Crutchfield has an ok selection, and always an entirely legitimate product, but very expensive. They include a warranty and "authorized dealer" status, which only count for anything if you have every bit of your install done by a professional and authorized shop.
Best buy is just pretty much terrible for everything.
And as for what you've heard about "Alpine" or any other brands, just forget it. It's all objective, and almost certainly misinformed. Most the name-brands are just fine in their performance and quality, and then terribly over-priced.