Proper length quarter wave whip bolted to your roof. Make sure that none of your connectors or studs are chromed, chrome is the bane of many antennas. Also for best possible results check it with an antenna analyzer for resonance, or if you can't do it yourself get a knowledgeable friend or pay a reputable cb shop to do it. This last step will change an overall good antenna setup to a great setup.
Everything else is a compromise, however, to minimize the compromise use as long of an antenna as you can mounted as high as you can.
All this is of course assuming that your looking for an omnidirectional or near omnidirectional antenna setup.
The DB