As things currently stand - FaceID.
I honestly like Android as an OS more than iOS and keep bouncing back and forth. (iPhone 6s Plus -> Pixel 2 -> iPhone XS Max -> Pixel 3XL -> iPhone XS Max -> Galaxy S10 -> iPhone XS Max)
In case you can't tell... I WANT to use an Android phone. I just can't find one that hardware wise fits the billing. iOS is just too restrictive for my wants/needs, but oh well. With my iPhone XS Max activated, I sit.
The S10's fingerprint scanner is a pain. Works most of the time, but when it doesn't it really gets me frustrated. It is also very slow compared to normal captive fingerprint scanners. And why oh why will Samsung not drop the damn curved displays? Accidental touches drive me crazy and it is impossible to get a screen protector on without either bubbles or dirt accumulating along the curved edge.
Please, please Apple. Let things loose a bit. Open up some API's for signal information. Let us install system wide tweaks. Widgets on the homescreens. Not being stuck into rows and rows of apps - let me move them where I want! Give us Dark mode (iOS 13?). And finally - add an Always on Display as an option.
Apple has the hardware down.. they just need to loosen some of the controls on iOS. How is the main OS layout the same as 10 years ago? 🙁