The former -made- the thread... so... good luck with that.
I love Mega Aggron, and while I think it will have to tip-toe around wayward Special attacks a lot, its being underestimated. Quoting common weaknesses as reasoning for a mon being unusable in ubers is kinda silly. All sorts of other mons have the same weaknesses. The calcs quoted earlier are still noteworthy even if they don't make Aggron a hard counter to the various mons listed, as many of those attacks will OHKO any other uber, just the fact that some of them will have to hit Aggron twice to have a chance to KO could clench games as many of them are not designed to take hits back. I think it'll definitely have a niche. If people use Blissey/Chansey in ubers I fail to see why they wouldn't use Aggron.
Chansey is also considered unviable, while Blissey really isn't great. Both also have
far better qualities as walls/support Pokemon (Natural Cure is superb for any defensive Pokemon, they get Softboiled instead of being forced to use the far worse Rest and Sleep Talk, Wish and Aromatherapy are amazing support moves with little distribution, etc). Both notably lack prominent weaknesses (the moment they had even 2 weaknesses to common special attacking types the way Aggron is weak to 3 physical ones, they would be unviable). Moreover, Ubers is still more the domain of special attackers (making Blissey better and Aggron worse).
Being 2HKOed or almost 2HKOed means Aggron has trouble switching in at all against many prominent threats, defeating the entire purpose of a wall (because walling is only walling when you can switch into a threat and stop it cold). Groudon is far better as a short term solution to physical attackers, because its lack of pressing weaknesses allows it to switch in and deal damage in return or phaze. Landorus-T is even better, thanks to a Fighting-resistance and a Ground-immunity. Giratina is the best choice for a lasting wall, with Fighting/Normal immunities and a Fire-resistance. If you can't switch in at all once you've taken a little prior damage (or if the opponent has entry hazards), you might as well just be revenge killing the opposing physical attacker with a Scarf Pokemon or priority. Walls are supposed to prevent having to sacrifice a Pokemon before handling an opposing threat.
Also, that list of calcs is not as noteworthy as it seems considering some degree of entry hazards are a pretty standard battle condition (trying to Defog every time an opponent sets SR gives offensive teams far too many free turns) and that a good number of those calcs neglected appropriate items. Here's something more realistic:
252+ Atk Life Orb Ho-Oh Sacred Fire vs. 252 HP / 252+ Def Filter Mega Aggron: 135-160 (39.2 - 46.5%) -- guaranteed 3HKO after Stealth Rock
252+ Atk Choice Band Ho-Oh Sacred Fire vs. 252 HP / 252+ Def Filter Mega Aggron: 154-184 (44.7 - 53.4%) -- 87.9% chance to 2HKO after Stealth Rock
Also consider the 50% burn rate which, if Sacred Fire burns, both prevents Mega Aggron from even nearly OHKOing Ho-Oh with any move, and can easily turn what would've been a 3HKO into a 2HKO.
252+ Atk Life Orb Rayquaza V-create vs. 252 HP / 252+ Def Filter Mega Aggron: 179-213 (52 - 61.9%) -- guaranteed 2HKO
252+ Atk Earth Plate Landorus-T Earthquake vs. 252 HP / 252+ Def Filter Mega Aggron: 136-162 (39.5 - 47%) -- 0.4% chance to 2HKO after Stealth Rock
252+ Atk Earth Plate Groudon Earthquake vs. 252 HP / 252+ Def Filter Mega Aggron: 139-163 (40.4 - 47.3%) -- 1.6% chance to 2HKO after Stealth Rock
(A low chance to 2HKO, but if there is even slight prior damage or a layer of Spikes this becomes a relatively high chance, and Aggron can only threaten a OHKO for Landorus-T if it carries Avalanche and has no chance against Groudon)
Whereas other calculations such as offensive Aegislash or physically offensive Giratina-O are irrelevant because they do not reflect common sets. Extremekiller Arceus on the other hand occasionally also carries Fire Blast or Overheat, both of which 2HKO with a Life Orb with 0- SpA.
Also keep in mind that it must Mega Evolve; before it does, it has a completely crushing Ground/Fighting weakness, no Filter and worse physical bulk, meaning that it has far more problems switching in before it Mega Evolves (and again, switching in against threats without much difficulty is an essential, defining trait for walls). Considering it's typically a bad Pokemon to lead with this gives it further problems.
Pokemon need some kind of relevant niche to call their own to be viable. Aggron is almost always just an inferior version of Giratina, which has access to Will-o-Wisp to cripple all but Ho-Oh and Blaziken and generally fares better against almost all of the above, while also sponging special attacks substantially better than Mega Aggron. Aggron might do a lot better against Rayquaza and Zekrom, but it seems a poor trade for faring worse against Blaziken, Lucario, Ho-Oh and the ubiquitous Ground types. Especially when all its retaliations have quite specific, limited targets (as opposed to crippling almost everything with a burn).
Also, Melee Mewtwo made this thread when the metagame was underdeveloped. He removed Aggron from the Viability Ranking List himself, I don't think he would object to locking this thread.
