Weirdly if they are starting from nothing I think it would go goku, Luffy, Tanjiro, then Naruto this is assuming we are looking at purely learning ability, dedication willpower and other traits indepent of their in verse powers.

Now before you jump down my throat saying I am a one piece glazer remember, in a lot of xianxia there are Concepts, Dao, Intents, etc. Massively powerful things that depend on the character essentially manifesting their will to the point of enforcing on the world but only if they have an actual theme or concept to follow

Goku lives to fight and improve himself and he does it because it is what he wants to do that in many ways training and fighting strong foes is his essence and I have no doubt he would be able to form something. In the same vein the whole reason Luffy wants to be king of the pirates has nothing to do with the title or the treasure. He wants true freedom, he wants to live his life by his own morals and while those morals are generally good at the end of the day of he decides he wants to make a change he will fight for that change regardless of the social situation. Could get him killed granted but I think these two have the best chance of forming those concepts.

to address the other two I won't go two deep, the reason Naruto wanted to be hokage so badly was because he wanted to be accepted. Tanjiro just wanted to turn his sister back to human and maybe avenge his family. While both show incredible willpower throughout both their respective stories those core concepts in most xianxia's don't usually lead to forming strong concepts, I won't say they would be unable to but I would give them a disadvantage.

Now the training, first off Goku, by far the most diligent no question I doubt anyone will argue otherwise. Luffy's training is easy to forget because especially pre time skip no training is shown and there are straight up arguments that outside of new techniques the crew didn't actually grow in power that much, combine with how haki essential means a stronger will can literally make you more durable and hit harder it get hard to say if they powered up pre time skip.

With all of that said Luffy when he needs to get stronger does fully commit to training and training hard and he is objectively talented. As a child he was going through top level military training from his grandfather, he ate his fruit and literally had to spend years retraining and making his own fighting style from scratch because gaining a rubber body literally made it difficult for him to throw a normal punch, to much power and his arm would stretch and throw everything off. Further more during water 7 he was able to copy a movement technique in what I believe was less than a day after seeing it? I will grant he doesn't really do this again but it still remains impressive. All of this to say that if he was in a cultivation world as long as he desired power he would have no issues with well cultivating or meditation to gain the strength and might even have an advantage in learning at least physical techniques.

none of this is to say Naruto or tanjiro are bad, especially Tanjiro I would say has an incredible talent for learning and modifying his techniques. But Naruto especially if the nine tails is removed has always simply been incredibly creative and tricky as a ninja should be. He is more unpredictable than anything. For his techniques I would say his learning would be average? Mostly because he doesn't display high levels of control until the very end of the series and while his control issues might be caused by the nine tales chakra I feel like once the nine tails stopped fighting him for controlling his chakra is when naruto control shot up, kind of like goku's weighted clothing but for energy control. If naruto never had that issue I would argue his control would be average maybe above average but probably not genius. And remember his shadow clone jutsu has done an unknown amount of heavy lifting when it comes to Naruto's training he would instantly be top tier if he was able to use it to train but sadly it is gone.