The Pelican is my favorite vehicle out of the series. Very original and unique space craft. It's flight ability is also pretty cool, I like hovering vehicles. As for a drivable vehicle, I say the mongoose. It's fast and fun to drive.
Above all.... *Imaginary Drum Role* THE WARTHOG!!! Nothing else brings you the awesome feeling of an all terrain Jeep heading over sand dunes mounted with a .50 Caliber Machine Gun ready to drench the hot desert sands with the cold blood and gore of your soon-to-be Overkill. Thats the definition of a Halo vehicle.
I would have to say my favorite vehicle for screwing around is the mongoose especially when im with my friends on a map that has hills and ramps and all that. I think the best for combat is warthog though
The ghost, it's the best vehicle for splattering! Don't for get it can shoot back and it's purple to boot.....