Has the company/robot been around for a while? If they have only been around for a few months then perhaps you should stay away. Anyone claiming to have the best Forex robot for sale should have a history of their robot being profitable for their buyers.
Do they offer a guarantee/refund? If not then this is definitely a reason not to invest in that robot. If they don't have enough confidence in their robot to offer any money back guarantee, then why should you show confidence in it by spending your money on it?
Do they offer support to buyers? Depending on your level of technical expertise, some robots can be tricky to set up and configure with your trading account properly. Make sure the robot you buy comes with sufficient documentation and tutorials to help you get started, and a way to contact the company if you have problems.
These are all important questions that you should be able to answer 'yes' to before deciding which is the best Forex robot for you. You may be able to think of some more yourself.
One other major issue you should be looking at is if a robot will be compatible with your broker. You see, Forex brokers know that trading robots can be profitable, so guess what they do::)