I like adjustable cages, so I can fit larger bottles in them. Topeak makes some (Modula, I think), but I felt like the latest version didn't hold as large of a bottle. Recently picked up an Arundel Looney Bin, and it looks like it might even hold a Gatorade bottle, which would be nice, but I haven't tried it yet.

Neither of them were chosen for their looks. If that were a priority, there are some attractive, bird-themed cages from Portland Design Works that I like the looks of.