I believe forming a visual presence on another planet lightyears from his location and fooling a powerful jedi user long enough to allow his friends to escape eclipses all those.
He's finally finished his training. Yoda insisted he'd be more helpful to his friends if he completed his training than if he ran off and saved them, and he didn't listen then. Yoda gave him his final lesson, he listened, and he's finally figured out he doesn't need to run off to save someone, he can do it from the comfort of his own hermitage.
I think, all this time, he still didn't fully understand the force. He was able to use it, like Vader he had a talent and didn't have to work hard to use it, so he never had to struggle to truly understand it.
Once he reached a true understanding of it, he became much more capable - and much less violent. It was only due to that understanding that he became one with the force - I suspect if he had died prior to this movie he wouldn't be a force ghost (which we have no evidence of, but I'm sure he'll be back - someone needs to train Rey - and Ben).