Suitability for the tasks it will be required to undertake.
Price.
Battery platform - ie. will it be compatible with existing tools or will additional batteries be needed.
Price.
Most reliable would probably be Hilti, but that reliability comes at quite a cost. Chances are you'll find the vast majority of contractors are using Makita, Dewalt or Milwaukee and they'll all swear that their chosen brand is the best. Unless you're planning on caning tools, chances are most in the mid range or upwards are going to be more than capable of whatever you throw at them.