I’d say Robin’s.
I like the story behind Zoro’s powerup but visually it’s still most of the same that came before.
I don’t like the story for Sanji’s but it’s the same visually.
(Yes, in colour they stand out, but it’s a black and white manga, that’s pretty irrelevant for the most part).
Kid’s is just… something everyone thought he’d be able to do anyway.
Law’s is pretty cool.
I like the theme of Luffy’s, I like what it does, I like some parts of it visually but not others, but I’m not keen on it being changed to a different DF.
But Robin’s is my favourite because I like the story behind it, I like the visuals, I like how she got a good fight after all these years. She stands out for being the pleasant surprise.