Antennas 101 taught me the best place would be to drill thru the middle of the roof and mount up top. That would put the antenna up high and also provide the greatest ground plane, but would be kinda ugly. Mounting on the roof rack would be iffy, you would lose some ground plane action, but if you ran a copper ground strap from the metal mount to metal on the roof, the difference would be negligible. Those would be best for radio propagation IMHO.
However, IMHO the rear mount looks best, but again for best grounding, run some copper to the metal on the body from the mount. Mounting on the rear of course will block some signal ahead of you, prolly measurable signal loss if your a perfectionist. Use a SWR meter to measure the diff, you will see it!
The front looks just OK IMHO, I prolly wouldnt put it there.
I'm coming from a amateur HAM radio background and the serious amateurs dislike any sort of "lip mount" such as you are employing. Same for those magnetic mounts you often see around. Not only can they damage the surface they are mounted on, but unless you run a ground to the metal of the vehicle, it is often impeded by the paint etc, and never makes for a decent plane.
The ultra hardcore (of which I am not one) say that if your not willing to drill into your metal to ground the base directly into its hole, you are not worthy of mobile operation (or i should say, compete in mobile operation competitions)! Of course that's an extreme attitude, but the science behind it is sound.
Thats my 2¢