If you are running the speakers off of the head unit, then I would suggest either Polk Audio or the Infinity REFERENCE series speakers. Both sound great on almost any system as long as you are just pushing them with the head unit and not an amp.
Now, if you are pushing these speakers with any type of amp, I would say look into the Infinity Kappa series or the Bouston Acoustics or even some of the JL Audio equipment. The Infinity Kappa series speakers sound really really good!
I have run Infinity Reference speakers in all the cars I've had and built up the system. Running purely off of the power of the deck, these speakers will be hard to beat no matter what the vehicle is. They sound great for not being a high end speaker. Also, I have been running infinity reference in my heep for over three years now, and with all of the water, mud, and snow that has hit them........they still sound great!
Pioneer and Kenwood........not for speakers. I like some of the decks they make (mosfet internal amps are nice because they rarely overheat under heavy load) but the speakers............
Get a good head unit, run the infinity reference series speakers, and you will be very happy. Or look at some of the polk audio stuff. They are also good sounding without an amp. Then if you need a little more bass, just add a JL Audio W-1 10inch sub........you will be golden........