A lot of people are still not aware of this but you could view and sort by payouts Steem tags, categories or topics by going to: https://steemit.com/tags
As seen above, the all time best paid topic on Steem are topics about: life, steemit, and surprisingly photography.
As for the most popular topic, I think we could sort it out through comments. The more people talk about it the more it is popular is it not? XD
kr or Korean are the most popular. Again photography too and posts about life.
My guess is that personal blog posts are the most popular topics on Steem as the majority of Steem users are what we consider "normies" or people who have little-to-no knowledge about how cryptos work. These people often blog about their travels, the food they ate that day and/or just the daily happenings in their life. XD
In fact, it can be easily seen even on the Steemit landing page.
The algorithm of the Popular tags are typically mainly influenced by the payouts of the posts accumulated having those tags. In fact, it doesn't need to be the primary tag as long as it is part of the 5 tags allowable in Steemit (though there are some dApps having more than 5).
You can see all of the data under https://steemit.com/tags and categorize/sort all of the tags based on the payouts (simply click the word Payouts and it will automatically sort the posts either in ascending and descending order.
There, you can see that #life has the most payout gathered and its not totally a debatable topic at all. Almost every post are almost about life and everyone shares their views about life.
In my opinion that would be the topic of cryptocurrency. I have no actual numbers to back up that opinion and it is based on my own personal anecdotal evidence of what I perceive on steemit. It also seems to be a topic that many questions posted on steemit seem to be based on.
If I am correct in my opinion it would probably be that earning steem currency is one of the reasons many joined steemit in the first place.
I think it would be anything around and discussing SMT's. The entire platform is looking forward to the development and launch of these tokens.
It was originally announced 2 years ago and has dragged ob forever it seems. Many users see this as the key to growth and the start of mass adoption. Tokenizing and setting up new communities from Dapps has everyone's interest and I think this topic is the most popular.