Mag mounts don't suck, it's just the antenna that's connected to it. Let's be more accurate and say that they aren't the best but that they are more precisely a compromise and a convenience. They can be used and removed rather quickly. The reasons for a mag mount can be multiple, to remove when not in use, to remove when parked in the city, so as not to draw attention.
The best antenna for me may be the worst antenna for someone else. I use the Larsen NMO-150/450/800 and NMO-2/70 antennas on my van. Yes, with mag mounts, I don't want them ripped off when I'm in the city, I don't want them to be an advertisement when parked there. It is always best to permanently mount any antenna to the body of the car, drill a hole, place the mount and route the cable. But when you want to remove it, it should be easily removed by you, not a criminal.
The best location for an antenna is on the roof, next best is the rear deck, the worst is the front deck (too close to the noises produced by the engine) and any antenna will do, just depends on what you want to hear and how much you are willing to compromise. The NMO-150/450/800 is a compromise antenna but it works well for my needs. There are many more antennas out there which do the trick, so it's best to explore the antenna wiki here:
http://wiki.radioreference.com/index.php/Scanner_Antennas
The wiki is a good place to look over antennas and their capabilities. Yes, size does matter in some cases, if you want a big antenna that hears Low Band real well, get a big one, an antenna for UHF don't need much. I have great success with a quarter wave on UHF for that range of frequencies. A quarter wave on VHF will work quite well from VHF through 800 with no major setbacks.