That is a tough choice since we know very little.
There's Setzo, Yangchen, Kuruk, Kiyoshi, Roku, Aang, and Korra.
Setzo is responsible for why the Fire is as strong as it is due to him becoming a diplomat. The Fire Nation was in ruins due to so many civil wars breaking out that nearly destroyed the nation. Since he focused solely on the Fire Nation the rest of the world and spirits suffered.
Yangchen focused solely on the problems on the human side. She brought the world in balance but completely neglected the spirits and took the human's side whenever there were problems most times. As a result, the spirits were angry and restless due to their both being taken away and destroyed in the human world. The world saw as her as a holy figure and would prey to her. She did regret not paying more attention to the needs of the spirits.
Kuruk had to deal with so many angry spirits that either killed humans or kidnapped them. Since there very few wanted to talk he was forced to fight them ultimately kill a few since Spirit Bending was not around back then. As a result, his spirit became corrupted that caused him immense physical pain, and shorten his lifespan drastically. He told no one but one close friend about the spirits so they wouldn't have the same illness as him, told no one about his illness as he decided to party to cope with the pain, and said nothing to imply things got this bad due to Yangchen. The world did fall into chaos due to his early passing though since the world relied so much on the Avatar to do everything for them.
Kiyoshi had a difficult life from the very start. From being an orphan to having someone else identified as the Avatar instead of her. Kiyoshi had the bring the physical world back in peace as well as deal with a few angry spirits. The world knows her as stone cold and ruthless when she was the complete opposite. She created the Kiyoshi Warriors to give the woman of her village a way to defend themselves and the Dai Le to protect the heritage of the Earth Kingdom in Ba Sing Se. Her only regrets were what the Dai Le turned into and not being able to have deep personal relationships due to how busy she was.
Roku appeared to not have that many problems other than keeping Sozin in check. For the most part from what is known his life was peaceful until he had to keep Sozin in check or deal with the problem that led to his death.
Aang was a diplomat in a world that needed a fighter. He ran away due to not wanting to be the Avatar. Aang had to learn to stand his ground and fight or the world would continue to suffer. He was going to ignore it until trouble came knocking and had no choice. The way he dealt with Ozai was a way none of the past Avatars knew about since Energy Bending wasn't around for them and to deal with some threats they had to kill them. If he never discovered Energy Bending when he did, chances are Iroh would have had to deal with Ozai.
Korra was a fighter in a world that needed a diplomat. She was confined to a compound in the Southern Water Tribe after a failed kidnapping attempt by the Red Lotus. The White Lotus didn't challenge her which gave her a false sense of confidence that she could handle everything, didn't stay on her to focus on her spiritual side and knew she had a problem with the spiritual side, and didn't try to explain why things were the way they are to as why she was sheltered from the world. Dealing with Amon had to happen since ultimately showed how badly the white Lotus prepared Korra for the outside world and how to deal with problems besides punching it. Korra honestly handled Vaatu well and was about to seal him away again. It wasn't until Unalaq came in and fused with Vaatu did she struggle. Unalaq had years of battle experience and at that point was a lot more spiritual than Korra. Vaatu even came out of Unalaq, reached into Korra, and took Raava out of her. Korra was really fighting both Vaatu and Unalaq at the same time in a 1V2. By Book 3 Korra was finally prepared by her spiritual side is a lot stronger and her diplomatic side. It took losing everything and questioning if she was needed to finally get to the point she was never prepared for.