I'll add this in for a SHTF vehicle, as I saw it a LOT when Hurricane Rita was coming: It should be diesel powered. When the stations were bone dry with no gasoline, many still had a supply of diesel. Ideally you'd have a diesel 4X4 from the Big 3, but I bet people would have traded any gas vehicle for even a lowly VW diesel, just to have the fuel to get the hell out. That said, I think the ultimate vehicle would be a decommissioned ambulance (painted either Evil Black, or a spray-can camo). I like all things .gov surplus, from rifles to vehicles. I see the Powerstroke Ford ambulances go for super cheap when compared to a like model-year truck. It would carry a lot of stuff, get excellent mileage, have fuel availabilty, and people will generally get out of your way, unless it has turned to absolute cannibalism in the streets.
Unfortunately, it's hard to convince the wife that I need one, as it doesn't have many other uses.