The best cordless impact wrench should be easy to use and well balanced and, obviously, needs to be capable of tackling rusted nuts and bolts with minimum effort on the part of the user for prolonged use. Mainly used for changing wheels, some double as a drill and screwdriver, which helps to justify the price.
A spare battery is essential for un-interrupted work and the impact sockets are a must for every tool. If you are a part-time user then these may be less important to you.
The larger wrenches are lighter on the pocket, whereas the lighter weight come in at a higher price but saves you a lot of effort, so if your arms need all the help they can get, these may be the ones for you.
These five cover both those that are the most cost effective and the easiest ones to handle.