Which Mega is overall the most viable in ou?

i've been asking myself this for a long time
i want to know other people's opinion
i personally think pinsir or scizor

There's no one viable mega in OU, it depends on your team archetype. Rain teams may want Mega Swampert, while balance teams may want Mega Medicham for instance. The same rings true for other tiers as well, sometimes you may want to run Mega Lucario in the Uber tier despite Mega Salamence being more widely used. Before picking a mega, it's best to consider your team and see if it can slot in pretty well with what it needs.

Right now in terms of "Best Mega" in my personal opinion are as follows:
in OU: Mega Lopunny or Mega scizor, Lopunny is a great mon to use against Offense due to its 135 base speed and access to Stab Fake out, Return, and High jump kick and it also gets access to the elemental punches ( you use ice punch to hit Landorus-T) it is able to outspeed most if not all of the unboosted metagame. Mega Scizor has a great ability in the form of technician as well as the movepool to use it with moves such as Bullet Punch, U-turn, pursuit Etc. its typing is also great Bug/Steel leaves only weak to Fire and grants it many useful resistances such as fairy, dark, steel, psychic, and an immunity to poison. it also gets recovery in the form of roost and hazard removal in the form of defog. Scizor may also use an Offensive set if the player wishes using swords dance, Bullet punch, roost, and another move.
there are many good Megas in OU however these stick out in my opinion in terms what they can be used on as well as viability.

\if we are talking all megas regardless of tier ( not including mega Rayquaza as that's the best mega hands down) the best megas would have to be in my opinion Mega Salamence, Mega Lucario, and Mega Mewtwo Y. Mega salamence has a great ability in the form of Aerialate and gets a good amount of high powered normal type moves to use this ability with such as Thrash, Return, Hyper voice, and Double Edge and with access to Dragon dance alongside its fantastic 95/130/90 bulk and 135/120 attack and speed and acess to roost it can easily find opportunities to set up and sweep also sporting a passable base 120 spattack it can utilize a special attacking set as well. Mega lucario having a really good ability in the form of adaptabilty( which means its stab bonus is x2 instead of 1.5) allows to break just about anything granted it has a great offensive typing with Steel/Fighting, an offensive stat distribution of 145/140/112, and access to both swords dance and nasty plot as well as really good coverage moves and access to great stabs such as Meteor mash, Flash cannon, Close combat, and Aura sphere for what one would need it to hit makes this mon unpredictable at team preview and able blow away whatever the player needs it to. Mega mewtwo Y is a really good special attacker. while its ability in my opinion is lackluster it has the pure raw offensive power with 150/194/140 and access to thunderbolt ice beam and psystrike allows it to hit just about anything and with the introduction of terrains, psychic terrain allows psystrike to OHKO just about anything if not then it will 2HKO or blast the threat with it's coverage move of choice. With a speed stat of 140 it is really hard to revenge kill outside of something like scarfed dark type like yvaltal or tyranitar if it lacks focus blast or aura sphere or ghost or dark type priority.
overall this is my opinion on this question

Scizor is the best Defogger in the OU and checks stuff that's normally difficult to deal with, such as Zygarde and Tapu Bulu. Venusaur is similar to Scizor in that it's mainly relevant for checking major threats, like Magearna in this case.

Mawile is a personal favorite of mine, as nothing can easily switch into it. It's a guessing game of what move you think Mawile will use, because it has the tools to wreck any check and it doesn't really need many moves in order to accomplish that, unlike other wallbreakers like Greninja. Medicham is another Mega that's a really good wallbreaker, but High Jump Kick's unreliability lets it down and it has a tougher time wallbreaking due to Mega Sableye, Mew, etc. Diancie is great at wallbreaking, can sweep with Rock Polish, and can get past most of its checks with moves like Endeavor. However, it has issues with four moveslot syndrome. It needs Endeavor and Sub to beat Chansey, but then it loses to most Steel-types. If it doesn't have Endeavor, it loses to fat stuff like Chansey. Not to mention it lacks priority.

Lopunny is arguably the best Pokemon to use in OU vs. offensive teams, but Ash Greninja, Tapu Koko, and Mega Alakazam are great at that too. However, Lopunny stands out from those Pokemon by having priority and slightly better coverage than Ash Greninja. It can be a set up sweeper with Power-Up Punch, and Encore is nice utility vs. passive Pokemon.

Pinsir sweeps teams with ease if it and its team manage to weaken opposing Skarmory, Celesteela, Electric-types, and revenge killers. It stands out from other sweepers by having powerful priority, so there isn't as much pressure to remove revenge killers, you just have to weaken them for Quick Attack. Pinsir is hard to prepare for in general because it only has a few checks, so some teams will practically autolose to it. The downside is that it requires entry hazard control.