I don't really know the names of vehicles but I think a helicopter, that can only be damaged by guns which use high caliber ammo, is the best.
An air vehicle is the best for travelling. You can move freely at a high speed and you won't have much encounter with other players. Even if you do meet players, they must have guns like grizzlies to shoot you down and that takes quite a bit of ammo to do so. You can just simply block up the heli with plates so players can't shoot you with stuff like maplestrikes.
A helicopter is better than a plane as it is far easier to land and take off, seeing that you don't need a runway. If you get one of those helis that have HMGs then sure, that's a bonus, but I don't see it as an effective way to kill people as ammo is hard to find and it's not easy to aim in a moving heli too.