In lieu of a more specific responses here, get the highest quality speakers you can buy from a reputable, higher-end auido dealer and ask them for a recommendation. Check first the ohmage rating for the speakers in place and match that so the amp is not overloaded by your new speakers. The ohmage is usually painted/stamped on the magnet and usaully be 4, 8 or 16 in value. Higher numbers of the new speaker is ok, lower than the existing means more resistance and therefore more load. I had my whole system swapped out 10 years ago so I don't remember what was put in place.
Also, you should ensure that the target speaker fits. This is more than the diameter but includes the depth and size of magnet. Another tip is you may want to avoid those speakers that are brighter sounding than the others. At first, a bright speaker sounds better, and companies know this so they tend to make their speakers bright to jump off the shelf in an audio show room. The reason to be careful here, is that the amps in our cars are not so good and harsh on the top end. Add a bright speaker and you may be very irritated and unhappy with the resulting sound, particularly as you turn it up with the top down. Good luck and post your results.