Gonna cover the rock mons themselves here.
Tyranitar: This thing has been OU since GSC, and it's not hard to see why. It has great offensive presence as well as bulk and utility. Whether it's a wallbreaker, a rocks setter, a sweeper, you name it, Tyranitar is something you must prepare for, and I don't see it leaving anytime soon.
Lords Dome and Helix (aka Kabutops and Omastar): I'm lumping these two together because they are so similar. These guys are best used in rain thanks to Swift Swim, being able to tear things apart with Swords Dance and Shell Smash, respectively.
Aerodactyl: Speaking of Gen 1 fossils, Aero is best used with its Mega Evolution, and it's not hard to see why. It outspeeds the entire unboosted metagame, barring Mega Alakazam which it speed ties with, and has solid power and Tough Claws to boot, although that ability really only works for its coverage moves.
Diancie: Probably the wonkiest stat changes upon mega evolving, Diancie pretty much has to go mixed, but with 160/160 offenses and 110 speed, that's not really an issue. Earth Power is a major boon to it, and it's one of the best megas in OU.
Terrakion: A bit removed from its glory days, it still has enough to keep it relevant, namely, strong STAB coverage that lets it put pretty much whatever it wants into the other two moveslots. SD+coverage move, SR, double booster, you never quite know what you're going to get when facing Terrakion.
Rhyperior: Gigantic HP, Attack, and Defense stats, but mediocre everywhere else. Solid Rock is nice, but Rhyperior's weaknesses are too numerous to last too long here. Still a decent wallbreaker, boasting more raw power than Tyranitar, Terrakion, or Tyrantrum.
Tyrantrum: Speaking of the dinosaur that's somehow a Dragon type, Tyrantrum went under the radar prior to getting Rock Head. This lets it blow things apart with Head Smash without recoil, making it a fearsome wallbreaker indeed. It can also use Strong Jaw, but, like Aerodactyl, can only use it for its coverage moves. It gets Dragon Dance to sweep in lower tiers, but Head Smash is really the only good reason to use Tyrantrum in OU.
Shuckle: Last but not least, a mon that has the weirdest stat distribution ever, but enough utility to give it a niche. Shuckle is one of the few viable users of Sticky Web, which greatly aids slower offensive teams that don't have a particular issue with Bisharp, and Sturdy (along with Mental Herb for a one-time Taunt blocker) ensures that it can get it up. Infestation is also nice, as it can trap and wear down random things that you may want weakened. Please don't use Contrary+Shell Smash, though.