Well, as of 2016 Bowman/Topps can no longer make NFL cards, so you don't really need to worry about 2016 or 2017 for refractors.
As for what is popular, it is mostly just personal preference. I like Blue refractors the most, but others may not.
As for what is popular, it is mostly just personal preference. I like Blue refractors the most, but others may not.
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