Choosing the best hammock cover depends primarily on what weather it will be exposed to and what kind of hammock you have. The biggest issue with outdoor hammocks is exposure to the elements, primarily moisture and heat. If you are in a particularly rainy or damp area, you will probably want a cover that is made of some type of plastic or other waterproof material.
Most hammock covers are made for hammocks that are on stands or that have spreader bars, but there are some that are made for hanging hammocks. It is important to get a cover that best fits your type of hammock. A cover for a hanging hammock will probably not be as form fitting as one made for a hammock on a stand, so make sure you check what kind of hammock your cover is made for before buying it.
One easy way to make sure you get a proper fitting hammock is to check with the manufacturer of your hammock. The company may offer a form fitting hammock cover or can suggest one that will fit yours. If you buy the cover at the same time as the hammock, they may even offer a discount.
Another factor to make consider is if the hammock cover has snaps to hold it on. A cheaper cover might just be for basic protection, and resemble a sheet without any extra snaps or ties. If you live in a windy area, snaps may be needed to secure the cover.
It is also important to consider how much you want to spend on a hammock cover. Cheaper ones will do the job if you just need to cover a hammock on an overnight camping trip, but if you have a hammock standing in the back yard you might want to spend a little more. Some hammock covers even have additional extras, like a pillow or padding. That way, you can leave the cover on while still using the hammock. Usually, more expensive ones will be made of a more durable material and will last longer.
A final consideration in choosing the best hammock cover is the color. Darker colors absorb more heat, so may not be a good choice if you live in a sunny area. Lighter colors will disperse heat better. You may want to find one that you find suitably pleasing to the eye, or that possibly matches your current hammock.