How about "What song do you like best" or "What song do you like the best" ?
Things you know vs. things you don't know. Kio (what) is reserved for an unknown something, presumably you know your favorite song; especially if it's one of a set. So kiu (which).
Well, in English, you would probably only use "which" in this context if there was some list to choose from, e.g. if you showed your mp3 player to a friend and they asked "Which of these do you like best?"
If you were asking a stranger for their favorite song, you would probably say "What's your favorite song?"
In Esperanto the question you would ask the stranger would still be a kiu question. Because, presumably, they would know which is their favorite song.
I don't think it is about what the stranger knows. I think it is about what the one asking knows. In the following example the child is analogous to the stranger. The parent asking.
Parent: "I just heard a loud noise" Child: "Something fell" Parent: "What fell?" Child: "A book" Parent: "Which book?"
Once the parent know the kind/type of noun the question switches to "Which?" But until they have that knowledge it is "What?" The knowledge of the child is irrelevant.
"Kio" is never paired with a noun. Similar to what Fred Capp said, the purpose of "kio" is to determine the actual noun, not which type of the noun.
When you already know the noun (such as "kanto/song" in this case) but you desire to learn which noun (or song), use "kiu".
Interesting! It can be expressed differently. https://translate.google.com/?hl=en#auto/eo/Amazing!%0A%0AWhich%20song%20do%20you%20like%20best%3F%0AWhich%20song%20do%20you%20like%20the%20most%3F%0AWhich%20song%20do%20you%20like%20more%3F%0AWhich%20song%20is%20your%20favorite%3F
"Kiun kanton vi ŝatas la plej"? I really think this translation is wrong. The others seem good.
Well, it would work if you added in the plej. Currently, it would be "You like which song?" or "Which song do you like?"
The sentence I listen is Kiun kanton li plej sxatas. I do not listen vi anywhere.
"Which song do you most like." I wrote this and it was marked wrong. I would use this in everyday language. Is it not correct? Perhaps a little archaic?
Duo uses a database, but that database doesn't have every iteration of every sentence in it. If you pretty much think that you are correct (enough to gripe about it here, at least) then go ahead and report it. If you are wrong, then it will fall into a dark abyss and never be seen again, but if you are right… You get a special little note from the Owl thanking you for improving the program.