Here is the competitive merit behind each theorymon:
Ghost / Dark Mega Banette
STAB Sucker Punch, STAB Knock Off, and neutrality to Knock Off, which allows Mega Banette to switch in against Fighting-types much easier. All of those are huge buffs, especially STAB Sucker Punch from a 165 Atk stat, allowing Mega Banette to act as a solid revenge killer while also MEvolving of the same time, somewhat mitigating the problem of lacking Prankster on the first turn.
Intimidate Rhyperior
Just to clarify, but Intimidate helps Rhyperior take super effective physical attacks even better than Solid Rock, while improving drastically his ability to tank neutral hits, such as Outrage, Play Rough, and Knock Off, all common moves. For example, max HP Intimidate Rhyperior always avoids the 2HKO after SR from CB Dragonite's Outrage, while Solid Rock Rhyperior with max HP / max Def+ gets 2HKOed ~50% of the time with SR up. Intimidate Rhyperior just doesn't care about physical super effective hits, unless they are Water or Grass attacks, and can also invest much more in Atk and support its team with both Intimidate and SR. Competition with Lando-T is always there, but the sheer physical bulk of Rhyperior (which allows it to counter huge metagame threats such as Mega Char X and Mega Pinsir way more effectively than Lando-T), Fire resistance, STAB Rock attacks, Ice Punch, and Fire Punch, are all good reasons for a team to go with Rhyperior. Here is a calc just to showcasese this thing's physical bulk with Intimidate:
-1 252 Atk Life Orb Terrakion Close Combat vs. 252 HP / 252+ Def Rhyperior: 203-242 (46.7 - 55.7%) -- 14.1% chance to 2HKO after Leftovers recovery
Bulletproof Metagross
The ability to counter Gengar and check most Aegislash sets shouldn't be underestimated. You also get to check Psychic-types such as Alakazam and Mega Gardevoir much better, which is a plus too, especially when considering how dangerous Mega Gardevoir is to a lot of teams atm. Definitely a change that would make Metagross worth using again
Competitive Azelf
The potential is there, but it's hard to make a conclusion about this. Could be very useful if Competitive ends up being reliable enough, or at least makes opponents thing twice before using Defog, but could also be ineffective due to Azelf's frailty and thus inability to switch into most Defog users.
That said, my vote goes to Intimidate Rhyperior.