My vote goes to CB.
The reasons why I think so is that Aggron's greatest asset, Head Smash, would be used most efficiently from a banded set (is Aggron really going to be using its other attacks often). In addition, it has the resistances and raw power to pull it off.
Voting CB.
Because of his awesome resistances, I feel like we should use a Rock Polish Aggron. Now, bulky grounds and even waters will switch in and force you out, all day long. What can we do to prevent this? Toxic Spikes and Spikes help a lot in bringing them into KO range, maybe even for another attacker. Roserade comes to mind, being able to set up Toxic Spikes, and switch into bulky waters all day(immunity to Toxic helps a ton here). Roserade has problems with bulky grounds, but Skarmory loves switching into them, as other than Gliscor with Taunt, and Swampert with Hydro Pump, they can't do a damn thing. With Skarmory on our team, we can switch out of Flygon(presumably Scarfed if it switches in after we Polish) and set up atleast 1 layer(which is all that is really necessary against most waters). Rotom-A would be an excellent choice as an anti-spinner, who can also set up screens if necessary(Reflect allows him to survive Scarf Flygon's EQ, Light Screen survives ScarfTran's Earth Power).
I will stop now since I know we aren't to this stage by now, but my point I was trying to get acrossed is that it may be difficult to support Aggron, but it is far more effective using him as our main sweeper than using something like CB Aggron + DD Tyranitar, who both get slaughtered by Earthquakes, Close Combats, and Waterfalls. Some sweepers, like Lucario, Infernape, Scizor, and the dragons(Dragonite/Salamence), enjoy having overlapping offenses, in order to break down their counters, but fortunately for Aggron, the majority of its counters rely on Choice Scarf, or aren't airborne. This allows us to play a more conservative game, abusing residual damage and the natural resistances, rather than just shoving him into battle and Rock Polishing immediately.
Granted, I haven't used Aggron so feel free to ignore my vote for:
Rock Polish
Arguments about Metagross' superiority as a speed boosting sweeper didn't really change my mind.
Metagross is a better Pokemon period.
It quite possible that it's a better Choice Bander as well (faster, higher attack, useful ability, access to Explosion) so maybe we can't do a Choice Band Aggron because it's again outclassed.
The Pokemon used for the CCAT so far have generally been underdogs that the community is going to try to make work with the proper team support.
So I don't think the fact that Agility Metagross might be better by some measure should necessarily rule out a Rock Polish Aggron.
Personally, I believe a choice item is a horrible tradeoff and I can't imagine building a team around a star player than cannot change it's moves without switching.
To exacerbate matters, Aggron cannot switch out without granting a free turn as Scizor can with U-Turn.
Choice Band has its merits but I don't think it appropriate for a project such as this.
Nonetheless, I'd be interested to see how the team evolves even if CB is ultimately chosen.
According to the poll, RP would be the winning set right now. However, many people have stated they wanted a revote, after seeing people making great points towards the CB set. Now after counting up the revotes, here are the totals:
Choice Band-9
Rock Polish-4
Subtitute-4
It gives me great pleasure to announce, that after the bolded revoting, CB is the winner, and the set that will be chosen for the basis of the team.
Now, let's discuss what the team style will be!