A large, windowless van of any sort from around the early nineties. Mostly because nothing looks better than a group of guys piling out of the back of a van with waves of smoke coming right after them...
that and the fact that the mystery machine was windowless except for the back windows and the obvious ones.
... In fact, fuck it. My perfect vehicle is the mystery machine. Now if only it came with Velma, I'd be set.