A.    Examples of which and what used for things:

What will you have to drink?

There's gin, whisky and sherry: which will you have?

What does it cost to get to Scotland? ~ It depends on how you go. ~ Which (way) is the cheapest or Which is the cheapest (way)?

I've seen the play and the film. ~ What did you think of them? Which (of them) did you like best?

B.     Examples of which and who used for people:

Who do you want to speak to? ~ I want to speak to Mr Smith. ~ We have two Smiths here. Which (of them) do you want?

which (pronoun) of people is not used alone as subject of a verb:

Which of you knows the formula? ('of you' is essential.)

Who knows the formula? would also be possible.

C.     which (adjective) can be used of people when there is only a very slight idea of restriction:

Which poet (of all the poets) do you like best?

what would be possible here and would be more logical, but what (adjective) for people is normally avoided.