I just wanted to voice my opinion that I heavily disagree with dropping Mega Venusaur from B+ rank to B rank. Right now the general consensus is that Mega Venusaur is horrendous in this metagame due to other bulky grasses such as Amoongus and Tangrowth having Regenerator so they can stick around longer and the omnipresent sand in the metagame that can cut down Mega Venu's recovery in half crippling its health down, along with the lack of leftovers. Now I do admit that these are its flaws, and the other bulky grass types I've mentioned do give it quite a bit of competition, but I still can't ever justify dropping Mega Venusaur all the way down to B. These issues regarding Mega Venusaur have been there since forever. What exactly changed that made Mega Venusaur that much worse now? Sand has always been common in OU, and Tangrowth and Amoongus both have been OU for quite some time now so its not exactly like the new metagame trends hurt it significantly.
Mega Venusaur has a bunch of distinct niches that set it apart from the other bulky grass types in the tier that such a drop is unwarranted. First off, Amoonguss and Tangrowth are both used as defensive pivots that can switch into a bunch of weak or resisted attacks and stay relatively healthy throughout the course of the match due to their splendid ability. Mega Venusaur, on the other hand, is played much more differently and serves as more of a tank role. You usually never want to directly switch Mega Venusaur into oncoming attacks unless its a guaranteed 100% safe switch, you aim to get it in properly given a free switch. Once it's in, its almost always doing something productive whether its hitting something extremely hard on the switch or beating the vast majority of the metagame in a 1 on 1 scenario. It's not supposed to constantly switch into attacks that chip down its health continously, it has other team mates for that.
252 SpA Choice Specs Keldeo Hydro Pump vs. 232 HP / 4 SpD Mega Venusaur: 114-135 (31.7 - 37.6%)
252 SpA Choice Specs Keldeo Hydro Pump vs. 248 HP / 92 SpD Amoonguss: 144-170 (33.4 - 39.4%)
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 232 HP / 0 Def Mega Venusaur: 112-132 (31.1 - 36.7%)
252+ Atk Choice Band Huge Power Azumarill Play Rough vs. 248 HP / 168+ Def Amoonguss: 132-156 (30.6 - 36.1%)
Even with no defense investment Mega Venusaur is more specially bulky than Amoonguss and has basically the same amount of physical bulk. However, the power difference between them is huge. After spore has been used off, practically any neutral Pokemon can switch into Amoonguss with ease, whereas for Mega Venusaur, between SR and Sludge Bomb's poison chance nothing besides Spdef Jirachi, Heatran, and Chansey really enjoy switching in on it. With Giga Drain's recovery and more than decent power you're able to beat a whole bunch of Pokemon 1 v 1, such as Landorus-T, Garchomp, and Scarf Tyranitar, that you definitely wouldn't be able to with Amoonguss.
There's also the fact that Mega Venusaur is actually pretty unpredictable at times compared to Amoonguss which is extremely linear in what it can do. You don't
need HP Fire for the last slot at all, especially given how common a partner Heatran is with it or using Heat Wave Zapdos which covers basically all the threats you want to hit with Hp fire anyways (skarm + Ferro + MSciz mainly). There's a bunch of cool stuff it can use for the last slot, like Leech Seed, Knock off, Earthquake, and one of my favs, Sleep Powder, so you can pretty much cripple any given Pokemon on your opponent's team and also utilize it to beat stuff like a boosted Clefable who wants to keep spamming CM vs you. MVenu is also much faster than Tangrowth & Amoongus and thus can easily customize its EV spread to outspeed stuff like Rotom-W and Suicune so they can't volt switch out or set up a CM first on you, respectively, and both will be severely weakened that you can finish them off.
Of course, then you have the obvious stuff Mega Venusaur walls way better than Amoongus/Tangrowth such as Mega Manectric, BD Azu, Fire Blast Mega Alt, non Zen Headbutt Mega Medicham, Serperior, TG Ice Beam Manaphy, HP Ice Thundurus, Crawdaunt, Weavile, and much more, but I'm assuming everyone knows this already...