It really depends on the map, players, and hogs available.
Overall: Driver
Death Island - Chain Gun
Infinity - Doesn't matter
Danger Canyon - Rocket Gun!
Blood Gulch - Driver (Any Hog)
Gep. - Driver (You can never get good shots with any gunner)
Sidewinder - Driver (I have a certain tactic at the beginning)
Timberland - Driver (Like Gep, few good shots)
Ice Fields - Driver (Like Gep, and Timberland you can't get many good shots)
Did I get every map? Hmm...