I voted for 2015 because I seem to have more cards from that year than any other year. Maybe it's because I stocked up on Viola and Lucy Li when I first joined. And Sapphira.
After that, I have 2017 with the next highest number, closely followed by 2016.
The year 2020 still has a ways to go, but it's trailing by a mile so far.
I would be fun to revisit this poll in a year to see how my collection has changed!