Still working on my thoughts regarding Mega Diancie and I'll definitely post about it once we have a good idea of where it is. I definitely think it's the best Mega in the tier by a comfortable enough margin since it has an array of new options, received an assortment of buffs, and partners well with numerous top-tier threats in this metagame, but whether or not it's A+ (I'd hardly say S currently; if Dugtrio takes its leave from this metagame that might be a different story) or just one of the best things in A remains to be seen. Sharpen, 4 Attacks, Calm Mind, and Rock Polish are all better now that Protect isn't mandatory and options like Nature Power and even the occasional Endeavor have made their appearances lately. It's definitely shaping up to be a top-tier threat due to its variety of sets and its ability to perform unique roles by virtue of its offensive spread, excellent speed tier, and numerous viable sets, ultimately preventing it from being compared to anything else in this tier. It's unique and it's good.
Speaking of cool and unique Megas that were introduced recently, we also have Mega Latios. Mega Latios is the picture-perfect example of what a viable OU Mega should be, for an OU Mega is splashable, performs multiple roles or a single role incredibly well, and provides tools that the base form cannot utilize normally. And even discounting the thing's Mega status for a second it shares the role of a Dragon Dancer with a handful of things in the OU tier and manages to differentiate itself from them all quite nicely to boot! Zygarde is the most versatile Dragon Dancer as it has numerous alternative movesets and boasts immaculate neutral coverage by virtue of Thousand Arrows. Mega Tyranitar is the bulkiest Dragon Dancer, has a very respectable offensive typing with good neutral coverage and has an impressive movepool and works incredibly well on Aurora Veil offense. Mega Charizard X is the hardest-hitting Dragon Dancer by virtue of its immensely powerful STAB options, high base 130 Attack stat, and incredibly good Tough Claws ability, allowing it to hit hard although it suffers from immense 4MSS compared to the others. Mega Altaria (overrated AF, by the way) has the best defensive typing out of all the Dragon Dancers, although it lacks a lot of the power and coverage it needs and didn't take overly kindly to the -ate ability nerfs.
Mega Latios is more specialized than all of these put together, though. Mega Latios has the incredible niche of being the best Dragon/Psychic Dragon Dancer with access to Waterfall as a coverage option, Levitate as an ability, and an incredibly versatile base form as long as it equips literally anything but the Latiosite in the entire Overused tier. This is worthy of endless praise, for Mega Latios is truly something to behold. Skarmory shudders in fear as Mega Latios fails to consistently 2HKO it after four Dragon Dances. Ferrothorn trembles in terror as Latios sets up Dragon Dances for six turns after it used Leech Seed, stacked its hazards, went out, got a burger and some fries, went to the gym, probably paralyzed something, and even used an attack and proceeds to avoid even being 2HKOed thereafter unless it runs Earthquake. Landorus-Therian cries in agony as it takes a whole 70% from a +0 Waterfall only for it to erase the Mega slot your opponent could've used on something meaningful with a single Supersonic Skystrike.
The best part is that it looks very, very similar to Mega Latias! It can attempt to replicate Mega Latias's roles to resounding success at failing miserably, for Mega Latios lacks the diverse support movepool and bulk his sister brings to the table. Literally all the immediate power Mega Latios brings to the table when performing this role is invalidated because Mega Latias sets up far more easily or runs Reflect Type far more viably.
And the best part? If it wants to hit hard without any setup, differentiating itself from both the Dragon Dancers and Mega Latias, he can still fail miserably because his non-Mega counterpart exists. Defog? Forget it; you can't run that and still hit hard as well. Choice Scarf, Choice Specs, and Trick? You forfeit those rights as soon as you clicked that item slot and typed in "Latiosite." Roost? Good luck getting walled by a good portion of the tier because you lost one of your coverage options running it.
Mega Latios is truly something to behold. No Mega that's actually in the OU tier can manage to exemplify what it means to be a good Mega by being the polar opposite of a good Mega quite like Mega Latios. I feel like people are really sleeping on it. And considering it's currently unranked and very few if any people have talked about it in a positive light thus far, I feel like they should continue to sleep on it. Please don't rank this garbage.