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.