i dont think anyone can really say.
is probably the best way as to tell the most popular distros, ranked by their page hits.
edit: whoops.. dunno if i answered this properly. i didnt really think about it, i thought it was another 'what distro best for me' question by someone wanting to know the most popular one so they would use that. but it looks like youv had linux for a while and use slack so im sure you coulda figured that out.. sorry