I have used EndNote, Papers, Mendeley and Zotero as reference managers. I want to use Emacs + Auctex + RefTeX + Markdown + Pandoc as my writer tools. What is the best reference manager to go with this?
I am not a Mac user, but as far as I know of reference management software, AUCTeX (Emacs LaTeX package) together with the reference manager RefTeX are widely used, particularly because this last automatically manage inline citations and also for workflow structuring, using the Org-mode. There is also JabRef, but perhaps you should try which fits the needs of your workflow due to compatibility.
Allow me to recommend you the following links on the topic.
I am not an Emacs user, but as far as I knew from colleagues using it, the RefTeX and JabRef reference managers work fine. I suggest you to try the best compatibility of one of them for implementing a workflow. RefTeX has been used with Org-mode for that purpose. Amybe Zotero would be useful also.