Window Hummingbird Feeders
While any hummingbird feeder is great to have in your yard, nothing can beat having the hummingbirds fly up to your window while you are inside. So in addition you to the same features you want in any hummingbird feeder, you want something that is either small or made mostly of transparent materials, so you view is relatively unobstructed.
Of course the tradeoff is that you need color to attract the birds, with red being a favorite color of hummingbirds, so it should be part of the feeder, as you won't necessarily want to put coloring in the hummingbird food.
Attachment to the Window - It's a good practice to clean your feeder once or twice a week and you may be more inclined to do so if it's right in your field of view, so look a the window mounting scheme. Many window feeders attach with suction cups, which should be relatively easy to remove often, but you may be inclined to use a hook or similar mounting scheme that just sits outside the window so that you can simply take it down to clean..
Mounting - A shade window is better, as the sunlight tend to encourage mold growth in the feeder.