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I still like Golem, so here goes in three parts:
Part 1: Abilities
Rock Head is perfect for flavor and Sturdy is the best of its current abilities. The redheaded stepchild here is its hidden ability. Not only is Golem's family almost never found in sandy environments, its also its worst ability by far. Let's replace it.
Given its appearance and dex entries, there are a few options here:
Aftermath is thematically appropriate. Unfortunately it doesn't activate on self-destruction, but that's a buff I'd like to see it get. As it is, the ability is too mediocre to really "buff" Golem.
Bulletproof is another ability that is flavorful for Golem's dex entries, which frequently cites it ability to withstand dynamite. Pretty good as far as abilities go, making it a good possibility.
Heavy Metal doesn't fit Golem but I thought its worth mentioning because this is the MEGATON Pokemon after all, whose 300 kg are now relatively light these days in terms of heavy Pokemon. 100% it would be better if they just retconned its weight.
Shed Skin is very dex-appropriate, which seems obsessed with telling us how our buddy Golem sheds its skin once a year. As an ability its fine, but Golem isn't a great user.
Shell Armor also fits as flavorful. Its an okay ability, but Sturdy is already better.
Solid Rock is the second great ability to show up on this list, but sort of draws attention to its inferiority complex with Rhyperior, and without being blessed by similar HP, can't make as good use of it.
Stamina: As a Pokemon known for its defensive strength, maybe it would be best to play into that trait.
In conclusion, in the absence of a new ability specially crafted for my boy Golem, I think Bulletproof is probably the best option here, so we'll go with that.