Features to Consider When Choosing a Hummingbird Feeder: (in no particular order)
Red Color The most attractive color to hummingbirds is red. Look for red in the feeder itself rather than relying on dye to color the sugar solution. Hummingbirds are very inquisitive and even just a little bit of red on the feeder itself is quite sufficient. (See below for more on red dye.)
Ant Protection Built in ant moats (check the feeder's packaging) or add-on ant moats solve most of the ants-at/in-the-feeder problem.
Bee Guards The most attractive color to bees and wasps is yellow. Newer model feeders from most manufacturers no longer have yellow parts. Look for hummingbird feeders that claim on their packaging that their shape discourages bees from reaching the nectar (usually found with saucer-shaped styles).
Built-in Perches Hummingbirds prefer to sit when they feed if they are able to do so.
Size of Feeder The smaller the better, until you determine how heavy the hummingbird usage is.
Ease of Cleaning There should be no little nooks and crannies in the feeder for mold to lurk. A dishrag, a small bottle brush (an old, clean toothbrush is wonderful!), and a clean pipe cleaner should be sufficient tools for cleaning. Also very helpful are the tiny brushes specifically marketed for cleaning hummingbird feeder ports.
Ease and Cleanliness of Use Look for feeders that do not require excessive twisting or snapping to be put together; this reduces the chance of sloshing sticky sugar water all over the feeder. (And your countertop, your shoes, your kitchen floor, your patio...)
Rain Guard Some hummingbird feeders with their feeding ports located on top of the solution reservoir may allow rainwater to get into the feeder and dilute (and possibly contaminate) the sugar solution. Check the feeder packaging to see if a particular model is designed to limit this problem. There are also "rain guards" available, metal or plastic disks meant to hang above a feeder, marketed specifically for hummingbird feeders.
Wind Resistance Feeders hung in very windy locations may spill and make quite a mess doing so; at least one manufacturer is making a saucer style feeder that can be pole-mounted.
Instructions Some feeders have the nectar recipe permanently imprinted onto the feeder itself.