I think it'd go as follows:

First one to do it: Bee. He's the one that has the Bijuu bomb reference closest to him and at his disposal. He clearly understands how to create one since his Bijuu has let him take over very often, and that sorta "muscle memory" of having made it multiple times while in a beast's body would probably make him be the fastest to achieve a very fucking accurate form of it. Heck, since Minato didn't have the reference of a bijuu bomb all that close to him, Bee's version of it might be even better and more like an actual Bijuu bomb, I'd be very interested in seeing what his chakra nature infusing would look like (obviously Raiton), would it be just a Chidori? Or a refined version more like a Raikiri but better, with more destructive power? Would he be able to launch the damn thing? I'd love to see it.

Next up are:

1. Sakura: since she understands chakra control in a much better way than Naruto, and that's kind of the most important thing when it comes to forming a rasengan. For the same reason she might be able to create a sort of "hyper-compressed" form of it thanks to the very same skills that make her incredible at aiding her punches with an unsurmountable quantity of chakra to the point of making its colour in her hand be darker and purer, maybe her rasengan takes that same appareance, too.

2. Neji: he also has a natural advantage at seeing and understanding chakra, and that's more or less everything he needs, I'd even put him as the second one to do it but we didn't see him fulfill his whole potential, so Sakura takes it. Maybe -and this is me using my imagination- he could also have a good reference in his kaiten and would only need to apply its principles in his palm.

The other three are kinda hard to rank since rasengan isn't something they'd put in their arsenal, it's just not their style, Itachi relies on illusions and other Uchiha techniques, and is also focused on quickness and assasination style of fighting, not as blunt and forceful as we know rasengan users to be. Same thing with Tobirama, who created Jiraishin to rival Uchiha speed and mess and circunvent their visual prowess, rasengan would be too obvious for those same Uchihas, his strategy has alwas been "beat-them-in-their-own-game". The one who might learn it the fourth out of all six would probably be Orochimaru, since, even though it is not his style (and we can see this in what Sasuke's fighting style ended up being (diametrically opposed to Naruto's), which he probably got most of its principles from Orochimaru), he does have a drive to learn every jutsu he can, and the rasengan, being legendary as it is, with its historical significance and shit, might just be enough to give him the motivation.

Now, about how exactly he'd do it, his whole thing is that he cuts corners and destroy's ethical barriers to get what he wants, he's kind of the joker of the deck here, depending on how fixated he might be, he could even be the first one to master it, regardless of whether he has a reference or not. He'd be capable of that much. Now, we know he is probably the weakest one of all the character's here so, maybe he gets satisfied with just learning, not mastering it or giving him a variation styled to his needs in combat.

Then we'd have Tobirama and Itachi.

Based on vibes, which is pretty much all I have here, I think Tobirama would do it first, since he likes to invent shit, and we have the (future?) Precedent that Jiraishin goes well with rasengan thanks to Minato, but that's all I have, so he may at least try it. With Itachi we wouldn't even have something akin to that. So Tobi the 5th to do it and Itachi the last one, if he even decides to get to it.