I'd like to make a case for Aggron being atleast B or so. Between Thunder-Wave, Stealth Rock, and Dragon Tail, I feel like Aggron is an absolute monster to deal with.
+2 252 Atk Mega Pinsir Earthquake vs. 252 HP / 252+ Def Filter Mega Aggron: 144-169 (41.8 - 49.1%) -- guaranteed 3HKO
Not just Pinsir, but T-Wave/Earthquake/Stone Edge/Heavy Slam deals with a lot of stuff:
-It can survive a +1 X-zard's Flare Blitz, paralyze it, and then do heavy damage with Earthquake
-Aegislash is faster, it will attack first (252/0 Mega Aggron can survive a Shadowball) and Aggron can OHKO with Earthquake
-CB Talonflame doesn't even 2HKO, is OHKOed by Stone Edge
-CB Staraptor does fuck all, Heavy Slam OHKOs after CC's defense drop
-Pinsir is OHKOed by Stone Edge, can't even 2HKO at +2
-Azumarill is 2HKOed by Heavy Slam, can't do anything back unless it's at +6
-Scizor and Mega Scizor do fuck all even with Super Power
-CB Terrakion's Close Combat doesn't OHKO, Earthquake does at -1
-Bisharp can't 2HKO at +2, EQ 2HKOs
-Tyranitar and Mega Tyranitar can barely do anything, are 3HKOed by Earthquake
-Heatran's Lava Plume doesn't OHKO, Earthquake does
-Dragonite is 2HKOed if Multiscale is broken, but if it's intact Aggron loses if it activates weakness policy. Can still use Thunder Wave
-Mawile wins if it's at +2 and has Fire Fang, but is otherwise 2HKOed by Earthquake
-Mega Medicham wins but still gets paralyzed and loses most of its health
-Mega Aggron can't do meaningful damage to Mega Gyarados but can still paralyze it
-Normal Aggron can also "win" various unfavorable matchups, like vs. Mega Charizard Y, so long as Sturdy is intact
For being able to check a ton of notable and extremely dangerous threats (including three S ranks and a lot of the attackers in A rank,) Mega Aggron should at the very least be ranked.
B rank minimum. It can also do a Rock Polish set, potentially sweeping a team while all priority attacks bounce off it, and can use Roar/D-Tail to "beat" any set-up sweepers it would otherwise lose against.
EDIT: Aggron also gets Metal Burst. I can't be assed to calc how well that would work but it might be cool.