Mega-Altaria
I don't really view mega-altaria as a surefire ban, but I feel it's worthy of a suspect
based on the way it can set up and sweep on so many types so easily. Dark, Fighting, Fire, Dragon, and Electric teams all provide ample opportunity for mega-altaria to set up and struggle to take it out.
Electric: Electric struggles with it's lack of poison or steel coverage. It has Magnezone, but all magnezone sets are outsped at +1 and unless you run air balloon or have a unbroken sturdy you're stuck relying on thunder-wave Thundy and hoping Maltaria doesn't abuse healing wish support or just switch out. Counter Ampharos is something I've been trying lately to deal with both Mega-Altaria and Mega-Charizard X, as you can bait an attack and live it without mega-evolving with moderate investment; but it's an incredibly niche set.
Dark: Mega-Altaria sets up on an upsetting number of Dark Pokemon; Mandibuzz, Tyranitar, Crawdaunt, and more. You mention Weavile as a check but Weavile can only finish altaria off; unless the Altaria isn't running enough speed EVs to outspeed at +1 (252 Atk Life Orb Weavile Ice Shard vs. 64 HP / 0 Def Mega Altaria: 151-179 (49.1 - 58.3%) -- 98.4% chance to 2HKO). Even then Mega-Altaria can switch out and come back with healing wish later.
Fighting: Again, fighting struggles to stop a mega-altaria at +1 and provides the opportunity for Queen-DDD to set up. Fighting has supply to scarfers who can outspeed a +1 altaria but neither terrakion or Keldeo can KO it, only slow it down with icy wind or hope for a flinch with iron head. Bullet Punch Priority exists but again mega-altaria is bulky enough to roost it off and can just switch, ultimately.
Fire: Hope that Heatran is a jolly scarf or has an intact balloon. And again, Mega-Altaria can just switch out. Rotom-H can paralyze it maybe?
Dragon: Hope you have a +spd scarf Kyurem-B, a unbroken multiscale, or managed to set up mega-altaria first.
Those are just the most lopsided match ups; mega-altaria can also set up easily on normal teams who can really just hope they have the right Ditto happiness. A pokemon's usage doesn't translate to whether it's OP or not; it might make it less of a priority however. I think Mega-Altaria is suspect worthy.