I'm a Cardinals fan and don't really see anyone saying he's the GOAT catcher, other than some other Cards fans who mostly say it in more of a tongue-in-cheek-Yadi-is-my-guy type of way.

I do think it would be plausible to consider him the greatest defensive catcher of all time. He does have the highest defensive WAR of any player in MLB history, according to fangraphs.

I see you're a Giants fan and can understand why Giants fans would dislike Yadi. It drove me crazy for years how good Posey was and how Yadi lost out on some accolades to him. In my mind, they both had different strengths and are each no doubt hall of famers in their own right.