I carry two.
One is a 48" or so square tipped spade, with a blade about 8" wide. Fit across the back of the trunk with an inch to spare, and it is a good compromise shovel IMO becuase the square tip is less likely to damage tires and it's narrow enough to get under move vehicles. Similiar to but longer than this:
http://www.homedepot.com/Outdoors-Garden...catalogId=10053The other is a Cold Steel spetnaz e-tool. I don't like hinges in my shovels, and while it isn't a very good hatchet, when I have to go someplace where a hatchet or machete might get an ugly eyeball, a shovel doesn't.
I won't carry a plastic shovel- they don't do anything but break when they meet ice. If I was going to get hard core, a grain scoop would be added.