I think that these subjective list questions are useful, so it would be nice to have a separate tag for them (as in this example: What is the best introductory Bayesian statistics textbook?).
No, I don't think there should be a best-of tag.
The question that you are describing will always be a community wiki question. The tag will not give any more information about the real content of the question, but instead be used to indicate the type of question. Because community wiki is already doing this (kind of) I think it is completely unnecessary.
Yes, a separate tag would be helpful. However, instead of "best-of" what about "list of"? This is because some of the lists, e.g. my blog question, aren't really 'best of', they are just lists.