I just had an interesting conversation with a friend (non-native speaker of English, just like me) who was adamant that "which" can be used as an interrogative pronoun in constructions such as "Which is your favourite actor" and "Which is your worst fear", regardless of whether or not there is a predetermined, limited set to choose from. To me, this seems very odd, since throughout school and uni, I've been told that "which" can only be used when there is a limited set to choose from; otherwise I should use "what".
Of course, I had to delve deeper into this, so I checked Cambridge dictionaries, where I found the following passage:
We use both which and what to ask questions. We use which when there is a restricted range of answers. We use what more commonly when the range of answers is not restricted:
Which is the capital of Liberia? Monrovia or Greenville?
What’s the capital of Liberia?
This quote would seem to imply that whereas the rule I've learnt is essentially true, it's not set in stone – or am I misinterpreting?
In short: I'd like to have any native speaker's take on this!