I saw these sentence and not sure 'is' or 'are' should be used below,
'While it is hard to say which of these factors is most important, ....'
appreciate for any replies,
If there is ONE that is most important, then it's "which is."
If there are a number of factors that together go in the "most important" group, then "which are."
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.