To see the option, simply go into any sub-category, for example you can go into the AMA sub ama-ask-me-anything/ and you will see the screen below. At the very bottom, simply change the "sort by" to "views" and keep it in descending order, and click go.


That's really it, you just have to do this by each category. So for the AMA category we can see that the most viewed (as of now) is the Bitstamp AMA ama-ask-me-anything/i-m-nejc-kodri-co-f ... t3253.html
Here are some other ones I found:
Bitcoin Discussion category most viewed: bitcoin-discussion/announcing-the-block ... t1601.html
Dev and Tech discussion category most viewed: dev-tech-talk/so-just-how-insecure-is-a ... t2978.html
Economics category most viewed: economics/charts-charts-charts-t334.html
Trading Discussion category most viewed: trading-discussion/gemini-exchange-sees ... t2548.html
Speculation category most viewed: speculation/bitcoin-price-just-hit-265-t1436.html
Beginners and Help category most viewed: beginners-help/overview-bitcoin-hardwar ... -t200.html
Protip: To speed things up, you can append any category URL with "?sk=v&sd=d" and it will sort by views in descending order.