I would like to say Japan. The three main islands (Honshu, Shikoku, and Kyushu) that was Japan back then were at peace from the 1630s to the 1850s. Not a single war nor a major rebellion in that 200+ years.

If someone knows of a longer period of peace, I would love to hear about it.

Sorry, but I'm going to have to take this question down. This is a poll or "throughout history" type question, which seeks answers through a broad range of time. They are not allowed on this sub because they tend to invite poor quality, unsourced answers.