I don't really know what the best vehicle is, and I doubt their even is one. In space, I think that the Imperial Lander is one of the best. It can carry multiple troops, can take out most enemy fighters with one round of missiles, has a powerful gun, has high "health", and can take out the capital ship systems fairly quickly. As for ground maps, I perfer not to use vehicles, so I don't have much experience with them.
(^Click the pic to reveal my...) | Republic Commando: -]RC[-AjaX
BF2142: -]RC[-Auxidus | www.redcellgaming.com (I really need a userbar, eh. >.<)