I do a lot of mountain biking and have had great luck with the Yakima "Big Horn" bike rack. It is one of the few hitch bike racks that have an attachment for the downtube of the bike frame as well as the cross bar to keep the bikes from swinging and hitting each other. I have a fairly expensive mountain bike and haven't hadn't damaged yet in transport (on the trail is a different story) even when carrying 3 bikes. The Yakima Big Horn also swings down so that the rear hatch can be opened without removing the rack.

I also have a Yakima rack for the roof of my car but the hitch rack is easier to use.