Depends on what kind of sound you are going for. If you want an all around quality system I would go with Bose or Boston Acoustics. If you want a higher grade system as far as bass and all this is concerned I would say JL Audio or Rockford Fosgate. Kenwood and Alpine make awesome decks but I'm not a big fan of their speakers. That is my opinion.
This may be way too much info if you're just looking to replace stock speakers...but I'll lay it out anyway.
A good sound system will have an overall frequency range of 20-20,000hz.
That has bass (20hz) and also hits the high frequency (20,000hz).
Speakers will range from starting at 35hz, and ending at up to 25,000hz. If you're not going to have subs look for speakers that are closer to that 35hz starting range. The higher you go the less bass you're going to have. Also remember that lower hz require more watts as well.
You can check specs outs at http://www.crutchfield.com/
Last but not least start going to audio stores and listening. It's your system and everyone is going to tell you something different. Decide on your system by the specs and by actually hearing it for yourself. Most stores let you mix and match decks/speakers/subs so you can get a good idea.
Boston Acoustics speakers are INCREDIBLE. VERY clear and have some decent bass as well. Better priced than Bose.
I too love Kenwood and Alpine decks but am not real impressed with their speakers. They aren't bad by any means but they aren't Boston Acoutics.
Look for a set of 3 way components (woofer, tweeter, mid-bass) You could also get a set of 2 way components (woofer/tweeter) and get a set of 8's for mid-bass and then run a set of subs for real bass.
Seperating your frequencies is VERY important in putting together good sound. Depending on the type of box you have for your subs, you'll get very different sound but a set of mid-bass will round it out and help ou not demand too much from your full range speakers (which should be playing the highs clearly instead of trying to do everything).
what hozer said is pretty much on the mark. I haven't seen a electronics manufacturer (kenwood, alpine, yamaha bla bla...) turn out a "good" speaker yet (although Yamaha makes some decent home spkrs)... electronics and acoustics are two different worlds of thought. To combine the 2 it takes a different type of thinking... I worked for JBL Pro division for several years seen a bunch of stuff that just literally floored me... but for each really good design, I saw 10 times as many bad.
Example... you know you've heard some serious bass when you feel your vision blurring and get a sensation of your head heating up.... the vibration can make you sweat! <took almost 7,000wrms from a RANE amp and 4 - 18" JBL's in a wall sized box to do it...> :lol We built a shitload of them to be used in a IMAX sensauround experience theater. With all of em crankin it felt like a earthquake!!
MB Quart makes very good sounding speakers. Focal does as well, however thats at the upper end of the price point. Infinity are good, though some claim the tweeters are a little bright on their components. Best thing is to listen to several pairs and get what sounds good to YOU!!