Not just "some media ranking", it was Nathan Myhrvold, a former Microsoft CTO, and a collaborator of his (chef Francisco Migoya), who have an upcoming 1,700 page book called "Modernist Pizza".

Myhrvold got hit culinary degree while he was still the MS CTO and previously published the massive culinary tome "Modernist Cuisine" which is a guide to science in contemporary cooking.

TL;DR: they rank Portland at the top because of the wide variety of high-quality pizza available:

“To us, it means there are multiple choices of great pizzerias, ideally in different styles,” he says. Chris Bianco, in Phoenix, might make sensational pizzas, for instance, “but he’s one guy. I wouldn’t call it a great pizza city.” Portland pizzerias, on the other hand, offer genius in multiple styles, from New York-style pies at Scottie’s Pizza Parlor to Neapolitan-style at Nostrana and creative, ingredient-driven pizzas at Lovely's Fifty-Fifty.