In my opinion the best speakers that you can buy for your car are very expensive but there are quite a a few brand for your choice of speakers. I for one enjoy the speakers of BOSE and JBL, my own car has JBL and the sound quality is amazing. Bose is really good, but the most expensive.
The best is a matter of opinion. It depends on how much you would like to spend and what kind of music you wish to listen to. I can recommend Infinity's reference series for as a good lower end speaker, but higher end entries like polk and rockford fosgate will be your best bet; make sure to get an amplifier.
There is a speaker by the company named focal that is in the line called Access that is wonderful quality coax speaker. I really like the quality and richness of sound. Make sure that what ever speakers you instal your battery and switch board can handle it.
It depends on what kind of car it is and whether it has the stock head-unit installed. Stock head-units may require some new wiring to get the the new speakers to work. Consult a professional audio installer or website to see what will fit with out modifications.