Without further ado, straight to the facts:
- last version with sane amount of system services, which can’t be turned off
- absence of [neural networks API] , which is used for telemetry, collection/profiling user data with subsequent uploading collected information to third-parties [for the first time technology was introduced in Android 8.1]
- full support of any modern browser [even Android 6.0 can handle this]
- ability to load legacy Android 2.x applications without any issues [Android 14 is already blocking such apps] I care about older stuff simply because of code quality. New applications tends to be badly written in 90% cases: glitches, performance drops, missing functionality and so on.
- correct Wi-Fi implementation with full support of modern ciphers
- picture-in-picture support
- proper implementation of Hi-Res Bluetooth audio
- significant improvement of performance comparing to Android 7.1.1 [loading time of OS/applications was noticeably reduced]
- ability to record voice call without any hacks
Android 9 is quite satisfactory also, but introduced first signs of deterioration.
This version could be an option if Android 8 is too old for your device and can’t be installed under normal circumstances.
Android 10 presented plenty of unneeded services, which can’t be turned off. Some additional telemetry services were added also. It surely can be considered as breaking point of Android operating system. So everything beyond this version is complete sh#te by default.
I recommend to use the most latest build of Android 8.0. Timeframe: somewhere around September 2018.
Early 2017 versions have much more bugs and glitches.