I use a Thule Classic 9022 for two bikes. A 4 bike model is also available. You won't want this rack if you're carrying $200 bikes (or less) from WalMart.
The easier a rack is to use, the more you will use your bikes. Visit a local bike shop and see what they have. There are many different configurations. Tray models are easy to use and secure. Bikes tend to swing on the hanging models increasing chances of damage.
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