For me its the ‘Knock Code’ that LG had on their phones (I really wish LG still made at least the V series phones)
Basically there was a four-square area and you set up a sequence of where you would tap to unlock the phone. That set of squares was only shown when you set up the code
Then, to unlock your phone, you would tap those areas in the sequence you set up (even with the screen off).
Fingerprint readers are nice, but I really do miss the knock code
Edit: did find this article with a way to do the knock code, but if done wrong, could brick your phone I guess.
Plus, article is from 2014. When I looked at XDA’s info on it (they also being the developers) it looks like development on it is over, but individual modules may or may not still be supported by their devs
Active Edge, which LG originally came up with and that Google adopted with the Pixel 2. Of course they dropped it after a few devices.
It was basically a button/key press that you could configure to trigger actions by firmly applying pressure with you hands around the lower third of your phone. It gave a very satisfying haptic vibration response based on the amount of pressure you applied and you could even set the amount of pressure until it was triggered. It had something magickal about it.
If you’re interested in the tech: https://www.idownloadblog.com/2017/10/19/google-pixel-2-teardown-ifixit/
This and 3d touch were really nice phone input improvements