You have to weigh the practical with the theory here.
The best place for an antenna is the center of the roof. It provides the closest to omnidirectional coverage you'll get on a vehicle, it's the highest point on the vehicle and when applied via a NMO mount is most permanent and trouble free.
Now that we've all agreed on that, it becomes a bit of a practical matter. How much height do you want on top of your car? I have NMO mounts in the center of the roof of our car and pickup. They both sport Larsen 1/4 wave antennas that have been around at least 25 years. The one on the pickup "pings" the garage door when going under it, but doesn't hit anything else. When we're going off on a trip, I put a 5/8 wave (physically shortened via a coil in the base) Larsen on. Again, an at least 25 year old antenna. There is a difference, but not enough to put up with the clanging around town.
I've been playing with 2m FM mobile since I was first able to get my hands on some old tube type Motorola radios with vibrators in the power supply. Basically it was order some crystals from National, add a few "gimmicks" to bring the frequency down from the commercial frequency the radio was on, and tune 'er up. The rest was antenna. I've tried a whole bunch of antennas, and this is by far my favorite setup.