Besides having a good antenna, and adjusting the VSWR as low as possible, antenna placement has probably the biggest impact. For the best all around coverage, you should place it in the middle of a large "ground plane". On a vehicle, that normally means the middle of the roof. The design of the antenna to keep the signal parallel with the ground and not radiating up into space is the next biggest performance factor. The problem is that most antennas only show the pattern as seen from above. Your best reception would be an 103" whip in the middle of the top. Not real practical. A good through the roof antenna that is grounded and mounted in somewhat the middle of the roof will make a large improvement. While magnetic mount antennas are nice, they simply do not perform as well.
Having said all of that, on the trail, since the distances are normally so short, a rubber ducky antenna will normally suffice.