Tekumel came first, but had little impact for the hobby overall.
Next was Greyhawk, with it's generic Tolkien Western Europe fantasy, which was huge and still maintains a large following.
What if the first truly successful setting by TSR had been something else?
Maybe Ravenloft as presented in 2e? Our hobby would have much more of a horror feel than the generic fantasy it has now. And it might have gotten more general horror fans involved in gaming.
Maybe the Scarred Lands? This setting has much more of a mythic feeling to it. It's almost a post-apocalypse. It definitely has a lot more flavor than the generic stuff. I actually really like this idea.
Maybe Forgotten Realms? Yeah, it's generic, but it got tons of support. Before all the NPCs became god-like, it was a very cool setting. I think that's where it jumped the shark.
Imagine if something like Eberron had been the baseline. It actually kind of fits what was being published in novels better than Greyhawk. Many older fantasy books combined tech with fantasy.
I think another cool setting would have been like Mythic Russia. Russian folklore is pretty damn cool.
Or maybe people just would rather keep Greyhawk be the first big dog.