Face can be tricked with a simple copy in paper of the owner face. Several phones suffer from that.
Finger is tricky. If the owner is sleeping, I can place his phone on his finger to unlock it.
Or if I want to steal, I cut his finger out.
Iris maybe is the most secure. Of course, someone who's a bit gore, can remove someone eye.
Nothing beats a memorised complex password.
I mean complex. Not the stupid easy ones.