Screaming in anguish at omission of José Manuel 'el Tarangu' Fuente.Instead, I'll list my own top-10 of pure mountain riders in random order.
Charly 'Grimpeur ailé' Gaul
Marco 'Elefantino' Pantani
Luis 'Lucho' Alberto Herrera
Lucien 'de kleine van Mere' van Impe
Gino 'L'intramontabile' Bartali
Federico 'El Águila de Toledo' Bahamontes
José 'El Chava' María Jiménez
Fausto 'Il Campionissimo' Coppi
Raymond 'PouPou' Poulidor
Julio 'La pulga de Ávila' Jiménez
I do like the idea of taking the top pure climbers of their eras as a yardstick, and then being able to judge the climbing of the GC all-rounders relative to those. Sadly today it's less feasible as the depth in the bunch meaning fewer time gaps are opened by flat stages, and the increase in the number of MTFs and concurrent decrease in TT mileage, means there's little reason the kind of rider who would be seen as a 'pure climber' and that alone in yesteryear wouldn't see themselves as a front-line GC rider (think the likes of Miguel Ángel López, Nairo Quintana or Mikel Landa).
Yes, the one-dimensional climbers may have been the romantic favourites, but they had to be truly special to actually win a GT due to the all-round demands on GC riders back in those days. It's why pure climbers from those eras who won GTs - van Impe, Fuente, Bahamontes - are particularly fondly remembered. But rating them directly against one another, Herrera won one Vuelta, while Quintana has won a Giro and a Vuelta and has higher Tour de France finishes - but I'm not ready to anoint Quintana the stronger climber than Lucho and in all likelihood never will be; that's not to denigrate Quintana who is clearly one of the best climbers of his own generation, but the differences in their respective palmarès are largely the products of the generations they raced in and a rider of that skillset has far more opportunity to win high profile races today than they did 40 years ago when Herrera arrived on the scene.
But for generations up to and including Armstrong's, it's a good way of judging the climbing levels of the big guns.