For dash speakers, this is my thoughts and opinion. Being in car audio for a long time there are a million ways to do this, but this is my opinion on the dash speakers.
If you are an amplified setup get something with a medium-high sensitivity and signal range. As the amp will crossover and send ample power to the dash speakers. This will allow them to play loud and have limited distortion. The doors and sub woofer will blend nicely.
Now where the odd ball opinion comes in. If you are not amplified and have no intention of doing so, get the lowest sensitivity dash speakers you can find. Everyone in the amateur music world just felt me say that and here is the logic behind it. The front door and dash speakers are parallel meaning they both draw power as well full signal from the headunit, there isnt a crossover or any way to regulate power from one to the other. So you need the speaker to do the leg work. So if your dash speakers have a high sensitivity they will be much louder than the door speakers, and you are thinking how is this bad. Well as they are louder at a lower volume, the mid range and bass from the front door speakers is greatly reduced because you are playing the headunit at lower speakers. If you turn it up the dash speakers will become quickly overpowered distort and sound lousy. You spent money to sound better not worse! Look in the older threads though you will see 1 thing stand out when people put in dash speakers, almost all of them will claim they lost a lot of low end. Its because they did as they arent at the same volume/power levels they were before so the big 6x9's are not getting what they want to really move.
There are a million opinions and options, I gave you 2. Are they the end all, not by any means but it gives you an idea of what will work better for certain applications.
Guess I missed a part. Sensitivity is how loud a speaker will get per watt supplied. So a high sensitivity will be louder than a speaker with lower sensitivity. So a 90db sensitivity will be louder than a 89db.