You can look at a team and say which charizard makes more sense on that team, since I'm right at least 90% of the time, I'd say that fails the unpredictable criteria. I spend most of my time in the low 2000s, people are smart enough to have well built, synergistic teams, but not smart enough to run something just for the surprise factor without giving up viability. I don't know if its a thing in the top of the top to run Y on a team that appears to be built around X, but I know its not anywhere else. If a team is running the Charizard that is less effective on their team, I look at it as a pleasant surprise, and while I often have to adjust my strategy, somebody using the less effective of 2 pokemon is never a bad thing in my mind.The fact that you have to play the 50/50 game with guessing what Charizard might Mega-Evolve into already fulfills one of the S-Rank requirements: Unpredictability and Versatility
Allowing a dangerous Pokemon like Mega Charizard to either set up a Dragon Dance or set up Drought safely is already a huge detriment for the opposing player. But that's not all. Once Charizard mega evolves, its respective Mega Evolutions demand different approaches. While specially defensive Pokemon are the go-to answers for a Mega Charizard Y, Mega Charizard X doesn't give anything about them.
Also Venusaur, Gyarados, Heracross and Garchomp? Sure they might play slightly different but the same Pokemon that beat their base forms can manage to beat their Mega Evolved form. Venusaur despite Thick Fat despises powerful STAB Fire-Type Moves and Ice Type Moves. Talonflame remains a deadly threat to Venusaur, mega-evolved or not. Gyarados does lose its 4x weakness to Electric Type moves but Rotom-W and bulky Water-Type Pokemon remain solid switch-ins into Gyarados. Rotom-W can either tank an Earthquake from Mega Gyarados and Volt Switch out and bring in a faster threatening Pokemon, such as Terrakion, or it can Volt Switch on the Mega Gyarados as it decides to Dragon Dance and bring in an appropriate answer, such as Ferrothorn. Heracross ... I don't see how it plays any differently. It still struggles to get past its usual answers such as Skarmory or Talonflame. Garchomp does hold some validity for S-Rank as physically Defensive Pokemon such as Hippowdon and Ferrothorn will not appreciate Draco Meteor or Fire Blast respectively.
Talonflame's a threat to every grass type, without exception. Venusaur can run a tanking or stall set similar to its mega, but the only thing I've seen it do is use Chlorophyll to gain momentum on a sun team. That role is something that is worth a rating, but if you were to rank Venusaur and Mega Venusaur together, nobody would even mention it because its so overshadowed by its mega.
Gyarados and Mega Gyarados have totally different checks. Bulky water types can reliably be used, but the mega can take electric and rock hits way better, while the regular takes bug, fairy, and fighting type moves better. This is the one I'm usually wrong predicting because they're both viable, and while you can't usually tell from team preview if its gonna mega evolve, you have to treat them very differently, so you should rank them differently. They fill different roles on a team. (Also, Rotom tanking an EQ? That is not something Rotom can afford to do at the best of times, and often its enough remove Rotom from the match. Just check the calcs)
Heracross attempts to sweep, mega Heracross wall breaks. They have similar checks, but are viable for totally different reasons.
Garchomp similar "problems". The regular is fast and strong, great for sweeping and revenge killing. The mega is not fast, but much stronger, great for wall breaking and tanking. They fill totally different roles on a team