Mega Venusaur B+ -> A
This may be a steep rise but from using it a lot, I feel like Mega Venusaur should definitely rise out of B tier which I dont feel does it justice. It single-handedly can win games on its own esp for teams unprepared for it thanks to its massive bulk and beautiful defensive typing combined with Thick Fat.
I generally feel comfortable bringing it unless I see.....
Mega Salamence (yea this isnt a good matchup period)
Charizard-Y (can stall out sun turns if needed thanks to 75% healing from synthesis in sun but otherwise zard does massive damage)
Talonflame (it can live a Brave Bird if it needs to and do decent damage back with Sludge Bomb but you will be taking pretty heavy damage that may not let you survive future hits from pokemon in the back)
Beyond its dual stabs Mega Venusaur's moveset can be highly unpredictable because of a wide range or move options which allow it to check its expected counters.
HP Fire: Hit Ferrothorn, Aegislash, and Scizor
Synthesis and Leech Seed: extremely strong sustainability options which are part of the reason why Mega Venu can straight up clean teams on its own if given the chance
Earthquake: Lets it hit Aegislash, Heatran, and not be deadweight vs Mega Gengar which would otherwise trap and abuse it
Sleep Powder: Lets it actually pose a threat to misc pokemon who may check it but also helps your teammates by putting any problematic pokemon to sleep
It's also not too difficult to support. An electric flying type like Zapdos or Thundurus actually has great synergy as it helps take out threats on my uncomfortable matchup list, so Mega Venu can actually still be brought to those matchups and thrive if one efficiently deals with them. Beyond that one can run a couple more mons to deal with potential gaps based on what Mega Venu's moveset actually is, but I genuinely feel Mega Venu is able to sustain and make enough of an impact on its own where its not always heavily reliant on team support, similar to other A rank threats
Running it on a dual mega team is also pretty dangerous as it can be benched when the matchup isnt ideal and you are free to use your secondary mega if you wish, but otherwise it can brutalize the opponent in most other matchups.