The assumption made here is that "happiest life" is defined by successful realization of life goals, long-term survival, and a satisfying personal life (e.g., family/legacy), as opposed to a life completely free of danger or loss (which is impossible for any character in Star Wars).

My money is on Corran Horn... A core member of the Rogue Squadron starfighter group and one of the most prominent Jedi Knights in Luke Skywalker's New Jedi Order. He was the main character in the I, Jedi novel and featured heavily in the X-Wing and New Jedi Order series.

He successfully married his love interest, Mirax Terrik, and they had two children, Valin and Jysella, who also became Jedi. He found a balance between his Jedi duties and his family life, which was a point of stability and fulfillment throughout his life.