Good Lord, you might as well as about the best car. There are as many suggestions and opinions as there are models.
Be more specific about what your needs are - Lo band, VHF Hi, etc. and let the flames begin.
I will start off with 2 just to add fuel to the fire;
a. Larsen makes a VHF/UHF/800 Mhz antenna which is favored by many, particularly those that live in urban areas
b. Austin's Spectra is another really good one
Keep in mind that scanner antennas are, at times, compromises, particularly so on VHF Lo. To get any efficiency without loading coils, etc. you need a rather long antenna for a vehicle. You can retune the Spectra for VHF lo - however the whip is rather short. It does work surprisingly well for such a small antenna on this band, as long as the antenna is firmly grounded. As an illustration, I had it on my old Camry attached to a Diamond heavy duty trunk mount, and could, on occasion, hear Howard county MSP on 39 Mhz from my home in Bowie - a good 1/2 hour ride (it was noisy, but it was there).
Mike