Get product collection based on custom attribute (drop down attribute value)

For dropdown attribute, We can filter using dropdown option ID instead of Label.

So your code is like below.

$productcollection = $this->_productCollection ->addAttributeToSelect('*') ->addAttributeToFilter('tag', 28) ->load(); 

Above 28 is the ID of the popular option of tag dropdown attribute.

I assume the 'Tag' is your attribute you created in the admin and the value 'Popular' is a option label associated with one of the option id with attribute 'Tag' not the value itself. The value might be an integer number internally used by magento.

As per the answer here : Loading a product collection by a specific attribute drop-down value , It is not possible to filter collection based on the option label, however you can filter the collection based on attribute option id, i.e. the integer value. You can get the option id for an option label, in your case 'popular' from the model.

You can use the code from the answer in the link I mentioned above. I hope it helps you.

PS: I did not add this answer in comment simply because I CAN NOT. I do not have enough reputation to comments. So, instead of down voting for posting an answer instead of comment, post requirement at meta stack exchange to give ability to comment with lower reputation.