Every variant of the warthog ranging from the base hog all the way to the Sangheili variant they're all great, my personal favorite is definitely the Gauss hog i don't know what it is about it but not only do i enjoy the look and sound its just a good variant game play wise.
Hannibal Scorpion. Putting a Spartan Laser on a tank is ingenious not to mention the regular turret was a Gauss Cannon? In the Halo Wars universe though, it has to be the Vampire. "Stop them in their tracks."