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None of these are really that safe. Iris is probably the weakest since there's really not enough data to get off of an iris to definitively detect a specific person. Fingerprinting has been foiled so many times by so many people in so many ways that I doubt anybody really thinks it is safe over just convenient. (We used these guys to replace patterns and password locks after all.) Finally facial recognition is exactly what Vaneet said. You need a very good camera with its best lighting conditions so it has the most data points it can get from you. Then the software has to be good enough to figure out all the minor details that makes your face different from someone else's. We already saw how bad this gets. Samsung's can be beaten with photos. Apple's can be beaten in so many ways and worst of all beaten by its own software like when the presenter on the day of this feature's release couldn't get it to work.
For now I'd say fingerprint is still the best of the three just because it's the oldest and has had enough time to be researched most. But it isn't safe. In the future face might be better just because there's more data and with higher res cameras like Mi Note 3 and Mi MIX 3 you can get a phone with so many data points that it can't be fooled as easily. For example photos do not work on these guys. Being Asian doesn't work on these guys. (More on this in a later and hilarious MIUI Resources article) If smartphone cameras can become as advanced as China's facial recognition which has already successfully found 3 criminals in a crowd of over 60,000 then I think face will take over.