Touch ID, says FBI forensics specialist Bob Moledor in an interview, though Face ID on the iPhone X is currently presenting problems, not least because the eyes need to be open.
Not that you would care about the authorities unlocking your phone when you are dead, but the story about the first recorded case of the police trying to unlock a culprit's handset with his fingerprints after his death, is a fascinating enough legal gray area. This trailblazing culprit in question is none other than Abdul Razak Ali Artan, the ISIS-inspired attacker who rammed his vehicle into people near Ohio State University, then got on a knifing run before he got shot dead, back in November 2016.
Mr Moledor said that they applied his fingertip to the Touch ID sensor on an iPhone 5S, but eventually couldn't unlock the phone, which was then sent to FBI's forensic labs to retrieve the info. Still, a precedent was set, and nowadays post-mortem phone unlocking it is evidently a common practice, tip anonymous law enforcement officers - to find the last dealer of an addict who overdosed, for instance. As for Face ID on the X, and upcoming 2018 iPhones, it's more complicated, according to agent Moledor: "I don't know that's been used yet. It's probably going to be same as using the fingerprint. As long as the subject is recognisable, it should work."