Quick answer: without a hyphen

This is because ever is an adverb that, your example, modifies the adjective 'popular'. There is no need to link an adverb and an adjective with a hyphen as you might with a compound adjective like 'free-range' eggs. This is covered more here Hyphens in compound adjectives.
There is a section on adverbs half way down the right- hand side under the 'Beware' section.

The exception to the adverb rule is when the adverb could be confused as an adjective (e.g. well or fast).

Yeah, my answer was full of typos, but you get the idea.