Voted for Art,Text contents,Videos, Vlogs.
I picked the three I did because long form (blog posts) writing, video and art posts all have larger communities, and or dApps behind them. Curation trails like curie tend to favor text and art, along with music, but perhaps not as much. Video has Dtube curation backing it up.
Whether or not any of that should be the best selling, I'm not so sure. The thing is, there seems to be quite a bit of interest in non-creative content, too, as in video games where the participant is playing or interacting rather than creating. Steem Monsters and now Drug Wars are doing quite well as far as participation goes, and while I've not played either, reports from those who have are by and large positive, and it sounds like they are happy with the time they are spending there relative to their possible earnings.
I don't know if video games in and of themselves hold the key, along with games of chance, but the answer probably lies in diversity, and bringing on the most amount of people by providing the greater variety of content/game/businesses.