Well, the obvious answer is Excadrill, being a Uber Pokemon and all. But unless you are Battling in the Uber Tier, Excadrill cannot be used in another Tier. So the true answer is Gigalith, despite the fact that Excadrill is of a superior Tier.
If you plan on using a Rock Type in the Tiers RU+, do not use Gigalith, as there are way better options, such as Rhydon in RU, Rhyperior in UU, and Tyranitar in OU. Heck, Golem outclassses Gigalith overall! I hope you get my hint :3