I just wanted to talk about what I believe are the best mega Pokemon for each of the most dominant playstyles right now, but make sure to keep in mind this is pending the release of several new megas that people are expecting such as Mega Heracross, which will likely be the new standard mega for baton pass.
Sticky Web Offense/Hyper Offense
Pinsir is the cornerstone mega of sticky web offense, as flying resists on offense are really scarce, and the most of the ones that there are (Mawile, Magearna, Tapu Koko, etc.) can all be handled with a swift earthquake. Swords Dance can make this thing a very threatening setup sweeper, and it has a bit of leeway when it comes to its filler move because it can also run Stone Edge for Zapdos and the Steel/Fighting types or Close Combat if you're really getting annoyed by Rotom-W, which isn't exactly the most relevant Pokemon right now. Obviously Mega Pinsir still has a crippling weakness to rocks and can still be checked by a few offensive threats, but the influx of webs offense is really good for Pinsir because a lot of its normal offensive checks such as Choice Scarf Garchomp and Tapu Koko suddenly can't deal with it like they can when webs aren't up, so you can be swept really easily if your team doesn't have a solid Pinsir counter.
Bulky Offense/Baton Pass/Trick Room Offense
Mawile is another great mega right now and is in my opinion the best mega for bulky offensive, trick room, and baton pass teams, which are 3 of the most common playstyles in the meta as of right now. Under trick room, Mawile is an absolute monster, being able to smash through most of the metagame with Play Rough and melting through Steel-types with its coverage moves. Outside of trick room, Mawile is still an absolute monster, being able to sweep through teams with an SD set with an insanely strong sucker punch coupled with the scariest play rough in the tier along with its amazing coverage movepool. Mega Mawile is basically impossible to safely wall and has two really scary threats in AoA and SD, not to mention that it has the ability to run rocks if your team requires it.
Stall/Balance (don't use balance)
Mega Sableye is honestly the only mega worth using on stall right now, and for good reason. Mega Sableye can basically control the hazard game for your team and set up the early game for you to do well going forward. The borderline broken (in the community's majority perspective) relationship this mon has with Dugtrio, along with its ability to deter common methods of dealing with stall such as taunt stallbreakers, makes it the optimal choice for stall as it provides so much utility and can overall be a really strong Pokemon to use overall. Additionally, Mega Sableye counters many Pokemon that would otherwise be quite a bit more annoying for stall teams to handle such as Mega Medicham and Stallbreaker Mew, which only furthers its value. I'm pretty sure that half of this subforum's competent playerbase could write up an essay on Sableye, but I'll just leave it off here before this starts dragging on.
I hope you guys enjoyed this post and found it insightful. Thank you and have a great day!