I'd really dig an MRAP, something with armor, mobility, and a usable gun on top. Imagine fighting off raiders by the dozen with a mounted machine gun as they charge you with lead pipes and knives. I can't find the link, but if you guys ever saw the EOD series (I know EOD uses JERRVs, but no one in Fallout uses IEDs, so no need for that kind of armoring) on whichever channel two years back, episode 6 or 7 had them surrounded by insurgents on the hills of the valley they're trying to clear and they mounted a camera on the gunner's helmet, it'd be really cool to be able to roll into the the Great Khans' canyon and have a firefight similar to that.
I'd also enjoy a vertibird, I know XRE has one, but one that doesn't kill you half the time you land. Something that you could set to hover to use a mounted minigun, think the Enclave Remnants in the battle of the dam, but whenever you want to take out large numbers of enemies with minimal risk.