I know you are using the definition of tyrant here to be "unjust ruler" as opposed to "absolute ruler". You can certainly have benevolent tyrants but I would argue that, without a constitution, you are by definition ruled by a tyrant. The USA has the oldest ratified constitution so that is a prime candidate for being considered the oldest stable non-tyrannical government. Of course, we are using different definitions of tyrant so you will not agree with my conclusion.