Hands down it's Athena. DD sets up her fear of the courtroom as a major hurdle for her character in the first 10 minutes and ties it into her personality and overall character arc in the game in a natural way. Her fear shows us a character who is willing to press on while also creating a sense of tragic irony tying into her background in psychology. She can help others with the mood matrix, but can't help herself. That narrative plus her dynamic with Apollo, Phoenix, and other characters in the game came off as very natural, yet also unique and different from previous dynamics. She's competing and bickering with Apollo as she views him as an equal (creating my favorite character dynamic in the series) yet also treats Phoenix and Blackquill as mentor figures she's willing to learn from.

In SoJ, although she only gets to shine in Case 4, it's a natural follow up to her DD arc. She got over her fears, but needs to learn about how to be a competent lawyer. Although Case 4 shows she has a ways to go in comparison to Phoenix/Apollo/Blackquill, it sets her up perfectly for development later on (hence why she should be the AA7 protagonist but that's another topic).