About M Venusaur I meant that sandstorm cuts its only reliable priority. Giga Drain can act like that too, but not always and recovering just 25% with Synthesis isn't enough for a mon that should act like an answer to threats like Azumarill and Keldeo. Also, burns are annoying for Venusaur as well, if you get your Venusaur burned, especially if sandstorm is up, it won't keep itself alive for a long time. The same can be said about Azumarill of course, which is supposed to beat Keldeo, but if it gets a burn it becomes pretty useless. However, unlike Venusaur even if Azumarill fears burns much more than it, it has a better matchup against the S rank and the A+ rank mons and that's why it's in S rank itself, for being an extremely useful mon that can check lot of threats. If you give a look at S and A+ ranks anyways, you will see how Venusaur can't really switch-in on against the majority of the mons that belong in these ranks:
S rank
Azumarill => Venusaur is supposed to be the best counter of it so yeah
Charizard (Mega-X) => Wisp variants are annoying, while the other variants beat it much more easily
Keldeo => It's an ok check, even if Scald burns and HP Flying from Choice Specs hurt
Landorus => Psychic and Earth Power do a ton, can't really switch-in on this thing
Thundurus => Check, but you should scout before if it isn't carring Psychic / HP Flying or you will be in trouble
A+ rank
Charizard (Mega-Y) => Fire Blast 2HKOs
Clefable => beat it unless Stored Power
Excadrill => EQ does a ton, can't switch in I guess
Ferrothorn => beat it with HP Fire
Greninja => Extrasensory is a thing, once again can't switch-in on this thing
Gyarados (Mega) => Check, even if +1 Ice Fang hurts
Heatran => completely walled if not carrying EQ, which is uncommon now
Heracross (Mega) => Pin Missile 2HKOs
Landorus-T => EQ doesn't hit like a girl, but check
Latias => Psyshock / Psychic
Latios => see above
Pinsir (Mega) => Return
Talonflame => Brave Bird
Terrakion => Stone Edge hurts, but it's a solid check otherwise
It's also true though, that lot of these mons can't switch-in on Venusaur either because how the good coverage that Giga Drain + Sludge Bomb + HP Fire has. In summary, after have read some posts I can see the reasons why Venusaur should be a S rank pokèmon, even if I would like to see more opinions about it :).
S rank
Azumarill => Venusaur is supposed to be the best counter of it so yeah
Charizard (Mega-X) => Wisp variants are annoying, while the other variants beat it much more easily
Keldeo => It's an ok check, even if Scald burns and HP Flying from Choice Specs hurt
Landorus => Psychic and Earth Power do a ton, can't really switch-in on this thing
Thundurus => Check, but you should scout before if it isn't carring Psychic / HP Flying or you will be in trouble
A+ rank
Charizard (Mega-Y) => Fire Blast 2HKOs
Clefable => beat it unless Stored Power
Excadrill => EQ does a ton, can't switch in I guess
Ferrothorn => beat it with HP Fire
Greninja => Extrasensory is a thing, once again can't switch-in on this thing
Gyarados (Mega) => Check, even if +1 Ice Fang hurts
Heatran => completely walled if not carrying EQ, which is uncommon now
Heracross (Mega) => Pin Missile 2HKOs
Landorus-T => EQ doesn't hit like a girl, but check
Latias => Psyshock / Psychic
Latios => see above
Pinsir (Mega) => Return
Talonflame => Brave Bird
Terrakion => Stone Edge hurts, but it's a solid check otherwise
It's also true though, that lot of these mons can't switch-in on Venusaur either because how the good coverage that Giga Drain + Sludge Bomb + HP Fire has. In summary, after have read some posts I can see the reasons why Venusaur should be a S rank pokèmon, even if I would like to see more opinions about it :).