I would love to say Chopper, Hornet, or Falcon but I have something else in mind...
The Revenant (Halo: Reach).
The Revenant was one of the few vehicles I actually enjoyed driving. It was swift, maneuverable, and had the bonus of a Passenger seat + It was pretty deadly (if used right). I can understand that the maps in Halo 4 were a little too small to hold such a vehicle (not to mention it was a little OP), but I would love for it to return in Halo 5.