Moonlighting as a Producer, Mixing and Mastering engineer I can fill out pages on this subject, but I'll keep it short and sweet. Since I don't know your car type, or what kind of music you prefer to listen to, I'm going to keep it generic to get a good overall balanced sound. Technically one speaker cannot produce the entire frequency range of the sound spectrum. So assuming you have a regular sedan, I would say get a decent set of tweeters in the front, placed at ear level, usually the A pillars, And for the doors ( assuming your doors have only a single speaker and don't have low range [bass] encloures built it ) get the speaker with the widest frequency range, between 12Khz to even 60Hz would be good. All this info should be available on the box or the website of the speaker in the form of a frequency response chart. Keep the speakers within the power output range of the head unit. Don't go too low or too high. ( Like your headunit will say 50w x 4 ) That means you can connect 4 speakers of 50w and be OK. Speakers too low can get blown out or distort the sound badly at higher volumes and speakers too high will mean you will really have to pump the volume to be able to listen at a decent level.
As for brand any of the popular and known brands should be OK. Since its a Kenwood headunit I would say go for Kenwood speakers and they could be better matched with the headunit. Pioneer is another good make for DIY car audio.