IMHO as an ex-car audio salesman and installer...
Blaupunkt is very bland and tends to be problematic for most users. Pioneer is okay, loud but not clean sound. Sony tends to be mellow but also they have a sound curve they use to create the "Sony sound" and if it's not your taste it sucks. Alpine used to be the all around king with a super flat response aimed at accurate reproduction of the media, however their quality sucks now and they fall apart too much. The finish wears off their plastic REALLY easy too leaving odd colored spots on your stereo face. Aiwa leaves too much to be desired and I tend to think of them as "Mickey Mouse". JVC is simple and for the most part inexpensive, leaving much to be desired. Kenwood tends to follow Alpine by flat response curves but their products tend to flucuate from year to year on quality, look, and features. Some years good, some bad.
My picks are Clarion and Eclipse.
However, your taste (like choosing a wax or sealant) is all that matters and you have to ask what you want from your deck. A deck that plays mp3's will allow you to play 200+ songs from one disc but... the quality will be quite lower ON A HIGH END SYSTEM. If you run stock or inexpensive speakers off the deck's power you will not be affected as much, but you can still tell.
JNGR. Like Beta, MD's never caught on because of Sony's execs. They overprice new technology that in both cases was vastly superior and chose not to license it out. Also, they priced them high very and in the US that doesn't get market share. In Japan, the most high tech, exclusive, and novel is what sells and they do well there. For reference Japan has had HDTV for decades because when it became available they bought it up. Over here we had to have laws passed (I'm not kidding) requiring us to switch over to Digital media (CD's, DVD's, and HDTV). Go figure....
ProbeGT good advice.