As suggested in the comments, if you are interested in some tag, there are several views you can choose.
You said that you are interested in the calculus tag. When you click on the tag name you get here - to the list of questions which are tagged with this tag. And you can choose one of several tabs to view/sort the questions: newest, featured, frequent, votes, active, unanswered. (Simply by clicking on the tab name near the top of the screen.)
If you hover with your cursor over the tab name, you will be shown a description what the particular tab means. (Similar thing works for many elements of the Stack Exchange UI.)
Since you say that you want to see the most popular questions, the following two tabs seem to be useful for that purpose. (In each case it is debatable whether these criteria in some sense correspond to the question "being most interesting/popular/relevant", but at least there is probably some correlation.)
- Votes tab show the most upvoted question. So these are the question where users thought that the questions are worth upvoting, which might be some rough criterion whether the question is interesting.
- I think that also the frequent tab is useful. This tags shows the questions with most links from other posts. This can also be considered in some sense as a criterion which questions are the most relevant. (But this probably corresponds more closely to "most frequently asked" question rather than "most interesting questions".)
I am unaware of a possibility to view most favorited questions in a given tag using the user interface. But you can try to use data explorer, as explained, for example, here: View Most Popular Questions by favorite count / views / votes. There is certainly a lot of space for improvement, but I have made a very simple SEDE query here: Most favorited question in a given tag.
Maybe it is worth mentioning that I have used
Tags like '%##Name##%' as a selection criterion in that query. So if you use it for the tag calculus, the question from algebra-precalculus will also be included. But I suppose that for most tags this not problem will not appear, since there are not too many tags with similar names. For example, I guess this will fork fine for multivariable-calculus or real-analysis. If you have some experience with SQL and you can create a query tailored to your needs or to the specific tags you are interested in.