For two or more bikes, the wave rack (with suitable coatings) is hard to beat:
The inverted U rack is another good choice:
Note the additions on these inverted racks. You can add all the frilly stuff you want, as long as you can lean the bike up against the rack and lock it.
Note both of these are just a little tricky to install: you need to leave access room all the way around the racks for the bikes. I've seen a picture of a wave rack right up against a wall -- cut the capacity by at least a half, because you can't put the bike perpendicular to the rack.
Now, for what to avoid. The wheel bender rack is cheap, easy to put in, difficult to lock a bike to, and, as its nickname implies, can damage bikes parked in one quite effectively.
