lol, the fact that something takes up the mega slot is a non-argument, especially regarding MegaVenu, who probably is the only really viable mega for defensive teams and therefor doesn't have any competition for that slot.I would suggest to drop Mega Venusaur from S-rank.
It has many notable flaws, such as:
-taking up the mega evolution slot
-no Leftovers
-vulnerable to status, especially burn
-bad STAB combination, Grass and Poison are literally the two worst offensive types in the game
-Taunt bait
-no reliable recovery, even Synthesis isn't that great with its low PP and Sandstorm being relatively common
-extremely vulnerable to Talonflame, the most common pokemon in the metagame
-4 moveslot syndrome. It needs Sleep Powder, Sludge Bomb, Giga Drain, Earthquake and Synthesis but it can only run 4 moves
While I understand it has many good qualities, these flaws are too much to ignore. Such a flawed pokemon shouldn't be in S-rank. Of course now I'm sure someone will inevitably post that these aren't real flaws and shouldn't affect its ranking.
I also seriously don't understand how people are finding this thing "unkillable". Even disregarding Talonflame and Mega Pinsir who eat it for breakfast, perhaps they're just not prepared for it?
Alakazam, Tornadus, M-Medicham, M-Gardevoir, most Taunters and pretty much anything with decent speed and Substitute that isn't weak to its common moves easily deal with it. I don't find M-Venusaur to be any more "unkillable" than Mandibuzz or Eviolite Chansey.
No lefties is migitated by other means of recovery (pretty much any MegaVenu carries two of them, seems enough to stay healthy). Status is annoying for pretty much any Pokemon and at least Venu is conveniently immune to Toxic and Spore. A Pokemon with 100/122 offenses can hardly be called Taunt bait. Bad STAB combo is only half an issue, because for one, it can simply use a coverage move (or put its counters to sleep), and two, it's a defensive Pokemon first and foremost, meaning that it doesn't give too many damns about the fact that his STAB combo is lacking. Remember how Ferrothorn was a top-tier threat last gen? Grass/Steel is an atrocious offensive typing, but you didn't hear him complaining about that while he was walling half the meta and then some.
In fact, we can also draw the line to Ferrothorn when it comes to things like reliable recovery, because mind you, Ferrothorn had Lefties and Leech Seed as its recovery, whereas MegaVenu always has Giga Drain and either one of Leech Seed and Synthesis, meaning in the end Venu might even have more recovery than Ferrothorn. 4MSS is kind of a problem, but not enough to undo its strengths. Weakness to Talonflame is a non-issue, because Mega Venu happens to be really good friends with Heatran and Rotom-W, both of which can stop Talonflame.