I'd say either Rosalina or Robin tbh, it seems to me that once people will have figured out all the possibilities offered by Ryu's vast array of moves, his metagame will stale, while Rosalina's Luma gives her a ton of room for metagame development, the Luma alone allows for a ****ton of things we probably still havent figured out, at the moment, she still remains a really technical character who requires a high level of insight and, to an extent, to know how to micromanage the Luma, if you can't do the latter, well, sorry but you need to in order to milk that potential. Robin, on the other hand, has some micromanagement as well with the whole durability, but what makes her really technical is how she needs to have a plan, how she needs to be reaaaaaally careful with how she handles her books, if, by any chance, she ends up with both Thunder and Arcfire out of uses, she's pretty screwed, her 4 Thunder variations, her Arcfire, her 9 Aerials also gives her some untapped potential. Her Levin also requires her to manage her smash/aerial uses, if she ends up out of Levin uses, she pretty much will be unable to kill for the next 7 seconds, something that can be fatal on a 1v1 Situation where you're both at high percent. tldr, that whole durability things forces her to always have a gameplan and to carefuly manage Neutral and Side B uses, while also having to remember when what book/sword will be thrown and be able to be used as a very powerful projectile.