imac 2011 21.5" (500GB HDD, 20GB RAM) - recommended OSX ?

Hi. I have an iMac 2011 21.5" (500GB HDD, 20GB RAM) and am planning to give this machine a new life. I have read somewhere that some of the upgrades may not be compatible and may cause WIFI/BT to not function properly. Some apps may also not work properly.

So which OSX is the most recommended and stable OSX to install?

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Author: @AskAccomplished2678