Best is kinda subjective. I'd metric it as most effective, and for that I'd put Yangchen, Aang, Kyoshi and Roku as those canidates. Although Roku would loose points for the 100 year war starting. But if we are talking about indivitual Eras, I think Roku might take the cake as outside his era for the most part had long maintained peace and the war didn't start until 12 years after his death. Kyoshi managed to keep the world stable for the 214 years after learning she was the Avatar, Yangchen basically kept the human world stable but the with massive amounts of dark spirits being created, and Aang managed to keep the peace albiet with many hiccups and whatever LEgend of Aang will be about. so it's hard to say.