Regirock

Great League: 3 / 5

Regirock has a titanic defense stat and decent overall bulk. Its typing however, doesn't do it many favors, as it results in more weaknesses than resistances. While access to Lock-On enables Regirock to frequently use its powerful charge moves just like its Regi counterparts, Regirock tends to struggle more due to greater threats of fainting from its numerous weaknesses. Over it's still a good candidate for Great League, but lacking the resistances of Registeel is what sets Regirock back. 

Ultra League: 3 / 5

Regirock's high CP maxes out not far above the Ultra League cap, giving it very solid bulk in the tier. It can work very well as a Rock-type attacker and fills the niche that this brings very well, with extra coverage on ubiquitous Steel thanks to Focus Blast or Earthquake. It has many good matchups in the tier, but it's also dead weight against a large number of major threats.

Master League: 3.5 / 5

Regirock is a solid generalist in the Master League thanks to its largely unresisted coverage and respectable stat product. It can act as a safe switch, denting just about everything that isn't Kyogre or Groudon, and can even be used to bait them out. Pairing Regirock with Palkia or Zarude can help take care of those issues. Although Rock is a poor defensive typing and leaves Regirock vulnerable to many common types, its impressive bulk and strong moveset often lets it fight back. While Regirock competes with Landorus when it comes to similar coverage, and with Mewtwo and Zacian when it comes to simply being a strong generalist, it can set itself apart from them thanks to its reliable matchups against Ho-Oh, Lugia, and Togekiss. Keep in mind that Regirock's fast move pressure is largely nonexistent, may struggle to efficiently finish off its targets.