While Anakin is the most tragic protagonist in this case, I'd go with Luke. He has the most fluent development out of these three, in my opinion. Starting out as an inexperienced, rather naive, idealistic young man, barely more than a teenager, becoming a more experienced, stubborn, reckless rebel, and ending up as a wiser, more composed Jedi. I think he has a great arc over the course of the OT. While his development may not be all that visible on first sight, I think it can be seen how he progresses and regresses, how his character is constantly shaped by his successes as well as his failures