The church that receives the most visits, surpassing the most visited cathedral, is Gaudí’s Sagrada Família but it is not a cathedral temple.
The cathedrals, in this order, are Córdoba, Santiago, Burgos, Mallorca, and El Pilar:
https://www.religionenlibertad.com/espana/260220/catedrales-mas-visitadas-espana-2025_116254.html
Four of them are extraordinary monuments, El Pilar, with the least artistic merit, but there no one goes for the art but for the Virgin.
The uniqueness of the Córdoba Cathedral is the Muslim footprint that makes it a unique monument in Spain
Santiago is the pinnacle of Spanish Romanesque but the number of visitors is not motivated solely by artistic considerations, which also, but above all by the Camino.
Burgos and Mallorca are two jewels of the Gothic but there are several of that style that by their merit it would be fully justified to compete with Burgos and Mallorca: Toledo, León, Sevilla, Palencia, Segovia…