(Wall of text incoming)
I'm pro-ban, and its been said quite a few times why but I'll repeat it. Mega Metagross has everything it needs to smash through almost the entire tier on its own. Very few things want to switch into its STABs (both having good base power), which coming off of 140 Attack along with Tough Claws boost will severely dent most of the Physically defensive non-resist pokemon in the tier. Along with with a high attack (and a move with a 20% chance to boost it further), it gets Base 105 Special Attack meaning that even uninvested its special attacks (pretty much Grass Knot) are going to hurt what they need to hit (READ: Slowbro, Starmie, Hippowdon, the things that can switch into its other moves). A lot. Did I forget to mention that Grass Knot also gets boosted by Tough Claws?
So with the above things in mind, let's ask what can reliably switch into Metagross? (I'm using the BreakMyTeam function of sweepercalc, the one I'm using is linked
here). We see that the standard set (Meteor Mash, Hammer Arm, Zen Headbutt, Grass Knot) is countered by Doublade, Alomomola and most variants of Scizor (Scizor can set up, though if Metagross gets a boost from Meteor Mash on the switch this becomes a different story, as it has a decent shot at a 2HKO non-fully defensive variants with Hammer Arm, while Scizor can't do much it it isn't already set up). Not much else. STABs and Grass Knot were addressed in paragraph above. Hammer Arm will let Metagross beat Skarmory when it Roosts, and will take a hefty chunk off of Ferrothorn. Victini doesn't take too much damage either, but its not that hard to wear down with its SR weakness. Cresselia can switch in too, but whats Cresselia actually doing to Metagross anyways? Lets not forget, this is what
one Metagross set
ALONE can handle. Except, unlike Mawile, "this" is virtually the entire OU tier. Not to mention it doesn't have to simultaneously beat all (is it 3?) of these things, because its got 5 other team-mates with it.
+1 252 Atk Tough Claws Mega Metagross Hammer Arm vs. 248 HP / 0+ Def Mega Scizor: 160-189 (46.6 - 55.1%) -- 69.1% chance to 2HKO (bulky SD set)
So, now I guess lets look at what can afford to stay in/revenge Mega Metagross? The list of things it can force out is pretty decent sized, choiced pokemon locked into a wrong move, Latias, Defensive Venusaur, non-Scarf Keldeo, and a lot of other pokemon I'm to lazy to type up, but that's not really a that big of a deal because a lot of pokemon are like that. The problem is, much like Mega Mawile, its really hard to revenge kill. In fact, not much outside of faster Megas (Mega Houndoom pretty much, most of the rest risk a speed tie), weather sweepers, and Scarfers with SE STAB, and Talonflame can reliably revenge kill it because of that really nice speed stat. Sand Rush Excadrill and Swift Swim Sampert can scare the 'standard' Metagross out, though you'd probably have to sac something to get them in not too damaged if flat out killed. Powerful STAB scarf attacks will also do a good job at denting it (think Diggersby, Landorus, Garchomp, Victini, Heatran), but the Earthquakes are pretty risky to throw around on Scarfers. Talonflame can handle non-agility sets, I give you that, but its really easy to wear down between recoil and entry hazards. Not to mention, Metagross still has 5 team-mates that should be able to handle the handful of pokemon Metagross can't. This is starting to sound a lot like Greninja to me, not going to lie. Except priority doesn't work because its not really all that bothered by priority not named Sucker Punch. Ah, now the Mega Mawile flashbacks are coming.
To wrap this up
(also TLDR), let's look at what the characteristics of an Offensive uber are
Offensive Characteristic: A Pokémon is uber if, in common battle conditions, it is capable of sweeping through a significant portion of teams in the metagame with little effort.
That sounds a LOT like mega Metagross to me. It isn't all that uncommon to bring it in (switch in on the long list of things it can force out, slow Uturn/Volt Switch, or after something dies if you really have to) and proceed to kill something. Its got great speed, excellent attack and a useable special attack, really good moves to for those attacking stats and ability, reasonable bulk uninvested making it difficult to revenge kill effectively, and (irrelevant but I had nothing else to add) a really cool design. Honestly, the only problem I see with Mega Metagross is that it lacks a recovery move. Then again, when has that ever mattered for an offensive pokemon?