Choice Band Aggron:
Choice Band Aggron is fantastic in this metagame. That being said, I think a lot of people struggle with something that I, myself, struggled with at first and that's how to support a Pokemon with such lopsided stats. Aggron's special stats and speed are barely 1/3 of its physical defense and 1/2 of its physical attack. There's also the two 4x weaknesses Aggron possesses to Fighting- and Ground-type moves. However, if you look closer, there are many Pokemon that can pair well with Choice Band Aggron. Here's a few:
Mesprit: Mesprit is one of only a few Pokemon in RU that is resistant or immune to both of Aggron's 4x weaknesses. Almost any of Mesprit's sets benefit Aggron in some way. The most notable would probably be Mesprit's TR set as Aggron just flat out destroys stuff in TR. However, outside of the Calm Mind sets (which can still weaken or KO Steels for Aggron) every Mesprit set has benefits for Aggron. The Offensive SR set can lay SR reliably and is one of the best offensive switch-ins to Fighting- and Ground-types. It has U-turn to bring Aggron in safely. All of Mesprit's choice sets carry U-turn on most occasions to get Aggron in safely. The Band set is especially intriguing since it usually runs Fire Punch and ThunderPunch to handle Ferroseed and Slowking to an extent. No matter the set, Mesprit is a great switch-in to one of the most reliable CB Aggron checks in RU, Poliwrath. Mesprit can also put HP Fire on any of its special sets to lure in and take out Steelix, arguably the best CB Aggron switch-in.
Slowking: Another teammate, another Psychic-type. While Slowking isn't immune to Ground-type moves (it doesn't even resist them) it can do one thing Mesprit simply can't; switch into Fire-types that like to revenge kill Aggron. Magmortar, Emboar, Moltres, Typhlosion; they all like to revenge kill Aggron once it has taken out something. Slowking also brings a resistance to Water-type moves to the table as well. Slowking can either slow down the opposing team with paralysis or form a fairly bulky wallbreaking core with Aggron by utilizing its Choice Specs set. Slowking, like Mesprit, can also provide Trick Room support for Aggron if desired. Slowking also has Regenerator to be able to switch-in more aggressively than Mesprit could ever hope to.
Druddigon: This seems odd, but there's one set in particular that can really help Aggron; Mixed Druddigon. Aggron struggles with Steel-types and very bulky physical walls and guess what Mixed Druddigon excels at forcing in and taking out? Yup, Steel-types and very bulky physical walls (bar Alomomola) Steelix, Ferroseed, and Tangrowth are all dealt with handily by Mixed Druddigon. Aggron can switch-in to opposing Druddigon with relative ease while Druddigon is able to resist Water-type moves and switch-in to most Grass- and Electric-types that may attempt to revenge kill Aggron. Flamethrower also deals with Torterra, a very underrated check to Aggron.
Uxie: Same typing and ability as Mesprit, but Uxie is much bulkier at the cost of offensive prowess. Uxie is pretty much the middle ground between Mesprit and Slowking. It takes care of Aggron's 4x weaknesses and is able to spread paralysis rather effectively. However, outside of running the odd HP Fire, Uxie can do nothing to help Aggron with Steel-types, Steelix in particular. That said, Uxie can provide dual screens, SR, paralysis, and TR support depending on the set, so its a solid supportive option for Aggron.
Moltres: The last one I'll cover here, but Moltres is a fantastic offensive partner for Aggron. Moltres levels pretty much every single physical wall that could ever hope to handle Aggron. Moltres's standard Roost + 3 Attacks set deals with everything Aggron wants out of the way. Steelix, Ferroseed, Poliwrath, Tangrowth, Torterra, and Gallade; they all get ripped apart by Moltres's main set while Aggron can easily switch-in to most of Moltres's checks, primarily Regirock. The combination of Moltres and Aggron will put the opponent under incredible pressure due to how powerful each is on opposite ends of the spectrum. Just look out for the shared Water weakness!