40 caliber Glock 23!
Pick a caliber you are comfortable shooting. For me, the 40SandW is perfect.
If this is for car use only, that is, you are not going to carry it, I personally would not go below 380 caliber at the extreme bottom end. As you step up, recoil becomes an issue. The most important thing in self defense is being able to hit the target IMO. Don’t overlook if you fire a round inside your car, there is going to be a blast and at night a good deal of flash to deal with.
Once you are pretty certain as to the caliber, then go to a good sports store and hand hold as many firearms as you can. How they feel in YOUR hand is very subjective.
Lastly, when you finally get your firearm, don’t lock it in your car, take it with you when you are not in the car.
Good luck with your choice.
FWIW, I have a glock 23 and a Bersa Ultra Compact. I like the Bersa because of the exterior safety but I am more accurate with the Glock. In my mags, I have Winchester Lawman 135gr JHPs, both guns use this round flawlessly.