Our countdown is officially underway with our first two Greatest Pop Stars of the 21st Century: Katy Perry (No. 25) and Ed Sheeran (No. 24)! Billboard's Andrew Unterberger and Jason Lipshutz are joined by longtime KatyKat Joe Lynch to remember the dizzying highs of Perry's early-2010s run, then Sheeran superfan (don't call her a Sheerio!) Walaa Elsiddiq comes on to discuss Ed's unlikely rise from coffeehouses to stadiums.

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