To begin with, I've left a decent number of mons unranked. I agree they all have a niche but they're outclassed a decent amount by other mons ranked above them and in some cases, I can't even think of situations where I'd switch the top 10 mons in my rank against these mons.
Now, to have a brief comment on my rankings I've valued utility over raw power when ranking. It's almost impossible to make fully viable teams without having half of the top 10 mons in your team. You can try, and you can even beat me without them but I doubt you can do it consistently. And yes, this means having only 1 Mega in the first 10 positions of my ranking, as I said when it comes to building a team it's almost impossible to do it without these 9 mons, they provide too much utility (Excadrill, Rotom-W and Slowbro), versatility (Clefable, Tornadus-Therian and Volcanion) or even both.
This doesn't mean the tier is in a bad state too as there are more viable playstyles than ever, but when talking about Megas you can pick from a wide pool of options, and they're all similarly good, with 3 of them (Lopunny, Diancie and Medicham) standing out from the rest for me.
Rather than commenting about every mon in the first positions of my ranking, because some of them don't even need explanation lol, I'm gonna highlight some of them:
A+ Rank
It burns. It paras. It toxics. It regens. What more can you ask for? After Rotom-W this is the Pokémon that punishes most in this tier, not to mention the fact it's almost impossible to OHKO them even with special attackers. You will attack, it will para you and switch out. Slowbro is the best switch-in to the likes of Lopunny, Metagross and Medicham. There's matchups you can even allow yourself to play with this while being Toxic'd or using Slowbro to scout your opponent.
I've learned to love this mon during SPL, it's impossible to have a way to play against every one of their sets and it's gonna be the Pokémon to rise the most in VR. Volcanion also burns, toxics and doesn't get OHKO'd, with a plus of being able to phaze (or just going Haze, but Roar looks more common right now) and hitting harder.
Offensive threat that has been on the rise lately, hits hard and can get past the most viable mons in the metagame, except for Clefable. But Weavile can beat them after Knocking, or by flinching or running Adamant.
I'm probably biased here but I love the utility this Pokémon gives. The access to Defog, Thunder Wave, Trick and Healing Wish is too good in todays meta.
I'm only gonna talk about the Static set as it is the standard now. This mon would be higher for me if it didn't have to rely on Static so much, it's awesome when it works but lackluster when it doesn't work. Volt Switch/Discharge/Thunderbolt, Heat Wave and HP Ice is an insane combination that gets to take advantage of teams most of the time.
A Rank
On the rise. Sand is having its time in the sun again, Tyranitar snuck into Invitational top 10 in usage with a 61% winrate (only behind Tornadus-Therian and Ferrothorn in the top 10). Tyranitar also provides a lot of utility, we've seen Choice Band, Choice Scarf, Assault Vest, Smooth Rock, Chople Berry and others getting usage in the most recent ORAS games, its access to a wide pool of moves also make Tyranitar stand out from other mons. It's not ranked higher for me because the best Megas in the metagame can take advantage of it easily and force it to switch-out, not before it cripples or picks one of the opponent's Pokémon in most cases.
Heatran is also a Pokémon that you can run a lot of different sets, but its best set is so good that it doesn't make sense to run variations like Choice Scarf, Choice Specs, Power Herb, etc. Magma Storm paired with Toxic can cripple every mon in the tier, Heatran's bulk, access to SR and phazing almost turns it into a powerhouse of the metagame. If it wasn't by the bulky waters or fighting-types in every team...
The god slug. Althought I like Gastrodon and think it's one of the best mons to stop volturn cores it's simply outclassed by the others bulky water in the tier. But you can't underestimate the slug, it has access to 2 good abilities in Storm Drain and Sticky Hold and can also be used as a Physical or Special Wall, it also has STAB Scald and Earthquake and the access to moves like Clear Smog, Counter and Curse.
A- Rank
Also on the rise, way less than the other Pokémon above them. I've used this Pokémon more than I'd like to in my most recent games. Zard-Y can beat the most used cores in the metagame easily, then you remember SR exists, Focus Blast misses and it only has 328 Speed.
Broken on paper. Falls short in practice almost every time.
Non-Mega is just better. Still, you can't forget about Slowbro-Mega when building or playing, we've seen it win a lot of games (probably more than it lost) in the last months. Its ability and great movepool make Slowbro a threat.
Toxic Spikes are great. It gets overpressured most of the time though.
B+ Rank
I was a hater once. Now I'm less of a hater. In all seriousness Talonflame can provide some utility and even sweep some teams in the current metagame, I've grown to like this Pokémon a bit more after building the team I used vs relous in Invitational Round 1.
Until some months ago I considered this to be an anti-meta Pokémon and abused it to win ORAS Seasonal, now that people noticed Thundurus it seems a lot harder to win games with it. I've ranked this higher than its counterpart because of the insane Special Attack it has, in some cases you can beat teams without having to set up. Thundurus also has a decent bulk that allows you to drop the Speed in exchange for Special Attack.
The Pokémon that was most hurt by the metagame development. I still think Keldeo is good and can abuse some of the most common cores, but having to rely too much on prediction doesn't help Keldeo's position in my rankings.
Similar to Gastrodon, but to a way bigger extent. It gets outclassed both as a ground-type and as a sand setter. Even though Tyranitar is better, Hippo has its uses.