The first key factor to choosing the best binary options broker is the payout portion offered. This is not always a simple comparison. Payouts will differ depending on the underlying asset being traded, and the expiration period of the option. In addition, the payouts could change. So where one broker was basically the best price, something may change afterward and imply that that is no longer the state.
The best a trader can expect for is to find the highest payouts for the types of trades he is most likely to perform and get a feel for which a broker offers the best terms most often. Again, demo accounts can assist in this respect.