A Prius. With the back seat half down, I fit just well enough. There are certainly other vehicles that would be better, but the Prius was adequate and nap-friendliness was only one of the criteria used to evaluate potential vehicles.

During my early consulting career, i'd often nap around 6pm before returning to work. I had different rental cars each week and almost any car works -- just recline the front seat all the way back and make sure to keep a pillow and eye-covers.

Whatever you do, don't get a Jeep Wrangler, unless it's a four-door model or one of the longer wheelbase ones...