Google TV: This is Google's TV operating system that runs full Android with all the bells and whistles including built in Chromecast. This operating system is probably the most versatile with access to the full range of apps and is most likely to be able to be updated to access any future streaming apps that may come out in the future.
Being Google, you can also integrate your TV into an existing Google ecosystem you may already have in your home, and with Google Assistant onboard, you can also say something as simple as "Hey Google, play the Mandalorian" or “Turn off the lights” and Bob's your uncle. Managing all your Google smart devices can easily be done in the Google Home app.
Android TV: This is a smart TV operating system based on Android and developed by Google. Simply put, Android TV is designed to bring the sorts of things you enjoy on your phone to your TV. It comes with a Google Play store which gives you access to almost all apps available.
Tizen: This operating system is used by Samsung. It is an open source platform and has been designed to suit Samsung smart TVs specifically. Samsung have access to almost all the apps available and are very quick to get to work on updates when new apps become available.
LG Smart TV (LG webOS): This is the operating system on LG TVs that’s quick and simple to use. It has access to a wide range of apps, but not all of them - Kayo being one. That said, LG is constantly working on updates.
VIDAA: This is Hisense's operating system. It was designed to simplify the traditional smart TV user interface and is easy to operate. It has access to a wide range of apps, but not all of them – Disney+ being one. That said, Hisense is constantly working on updates.