In terms of top Megas, I think three in particular really stand out: Venu, Metagross, and Alakazam.
Venu is solid against the wave of Electrics and Tapu Bulu (which is quickly becoming more popular), in addition to dealing with others pretty effectively like a good number of Aegislash variants and Pheromosa, which it only checks due to U-turn but can 1v1 and switch into to take a hit if absolutely necessary. I'm definitely getting shades of XY Venu while using it, especially because Aegi is keeping Psychics in check; however, I still think Venu will continue to be effective if/when Aegislash is banned. One of the main, and perhaps most underrated, reasons that I think Venu is better in this generation has to do with the burn nerf. I feel a lot more comfortable using Venu against Rotom and Waters in general because getting burned by Wisp/Scald isn't nearly as detrimental. It also fits really nicely in cores with some of the great new defensive Pokemon like Toxapex, buffed Mantine, and Celesteela. Venu is especially effective because Latios isn't as common atm with the massive influx of Aegislash and Fairies in general, generally meaning that offensive teams tend to have a harder time forcing it out. Furthermore, with the return of Aegislash and addition of Celesteela, there are more solid Psychic resists to combat the offensive Psychics that may be sent to break it.
I think Knock or at least Leech is pretty much mandatory at the moment for Venu because you can't be giving Aegislash and Landorus-I free opportunities. I've appreciated defensive investment (just using the 248/96+/rest spdef spread) just because there's a lot for Venu to handle in the types of defensive cores I'm referring to. However, offensive is still very solid, particularly for more offensive builds that still want to make use of it.
I'm not exactly too sure why people aren't using Metagross more. EQ is perfectly fine for covering Aegislash if that's really the issue. Mega Metagross is powerful and not prepared for nearly well enough by many teams. It can run priority, which is very valuable in a metagame populated with Pheromosa, Greninja, and Tapu Koko. It can break the Water+Grass defensive cores that people are using. In general, Mega Metagross is all-around very solid. It has great power and coverage, access to priority, and a fantastic defensive typing to check relevant threats like Scarf Tapu Lele, Tapu Bulu, Mega Alakazam, and Kartana w/o Night Slash. The Mega speed buff applies pretty notably to Metagross, as it didn't necessarily have difficulty before but certainly was inconvenienced by the process of having to Mega to gain its speed. The flexibility and utility that Mega Metagross offers to teams, especially with certain other Mega options off the table, is very valuable in the current metagame.
Probably will be even better in a metagame sans Aegi(/Phero), but it's still very solid atm. Shadow Ball can dent Aegi enough anyways to where it's discouraged to come in and outright beats variants that don't run Shadow Sneak. Mega Alakazam benefits from the Mega speed buff as well, giving it immediate access to the Speed it boasts over Tapu Koko and Greninja. Particularly with Mega Lopunny no longer competing for its slot, Megazam is very effective in the current metagame. It especially enjoys its new partner Tapu Lele, as Psychic Terrain blocks priority revenge kills and makes it absurdly strong. Think of it as Specs-boosted Psychics from a Pokemon with 175 base spatk and 150 base speed to put it into perspective. I think the calcs really illustrate just how strong this is, so I'm just going to list some here.
252 SpA Mega Alakazam Psychic vs. 252 HP / 0 SpD Landorus-T in Psychic Terrain: 333-393 (87.1 - 102.8%)
252 SpA Mega Alakazam Psychic vs. 252 HP / 16+ SpD Mantine in Psychic Terrain: 186-220 (49.7 - 58.8%) -- 74.2% chance to 2HKO after Leftovers recovery
252 SpA Mega Alakazam Psychic vs. 252 HP / 72+ SpD Clefable in Psychic Terrain: 255-301 (64.7 - 76.3%) -- guaranteed 2HKO after Leftovers recovery
252 SpA Mega Alakazam Psychic vs. 252 HP / 0 SpD Rotom-W in Psychic Terrain: 261-307 (85.8 - 100.9%) -- 87.5% chance to OHKO after Stealth Rock
252 SpA Mega Alakazam Psychic vs. 0 HP / 0 SpD Genesect in Psychic Terrain: 144-171 (50.8 - 60.4%) -- guaranteed 2HKO after Stealth Rock
252 SpA Mega Alakazam Psychic vs. 0 HP / 0 SpD Tapu Lele in Psychic Terrain: 123-144 (43.7 - 51.2%) -- 8.2% chance to 2HKO
Psychic is better, but I just wanted to throw this calc in there for the sake of context:
252 SpA Mega Alakazam Psyshock vs. 252 HP / 252+ Def Eviolite Chansey in Psychic Terrain: 325-384 (46.1 - 54.5%) -- guaranteed 2HKO after Stealth Rock
This kind of power is insane...I mean obviously you can't always have Psychic Terrain up, but when you can get it up (I reckon that Tapu Lele will end up being one of the Tapus who is pretty effective with Terrain Extender anyways just due to all the fast frail stuff it supports, which makes this more flexible) or an opponent sets it up (fairly common), Mega Alakazam can wreak havoc. The combination of fantastic speed, power, and coverage with its destructive powers in Psychic Terrain and general benefits from the Mega speed buff make Mega Alakazam one of the top Megas and among the upper echelon of Pokemon in general for this metagame.
