Seismitoad: Honestly this needs to drop to B+, but let's talk about A- now. It's definitely not the preferred bulky Water-type everyone chooses (Alomomola, Slowking, and Blastoise are more used), and as a Rocker it's also not preferred, as a rocker that gives free switchins to Grass-type is mediocre. The metagame is pretty harsh to Seismitoad, as it has tons of offensive threats it can't check (Fletchinder, Medicham, Sawk, Venusaur, Virizion, Sigilyph, Meloetta), and having no reliable recovery makes it a sub par Pokemon in the meta. I'm on board for A- but I recommend B+ for Seismitoad.
Venusaur: This Pokemon is one of the best Pokemon right now. It's best set in my opinion is Sleep Powder Synthesis dual STABs. Of course, it's checks are great checks and are good Pokemon in general, but you have to keep in mind that Venusaur can put them to Sleep which gives them a free turn. This free turn will be huge in the game if they have a wall breaker like Sawk, Medicham, and Emboar, as you'll struggle finding a check for that. Escavalier, which will get better once the suspects are banned (should be), will be able to counter Sleep Powder Venusaur. But we all know that Venusaurs tend to run HP Fire as well to beat Escavalier, so even though the latter set is worse, it's still a thing which will make the opponent hesitant to switch Escavalier in Venusaur. Yes, Venusaur has good counters, but it can take advantage of those counters, which makes checking it even hard. Venusaur should definitely rise to S rank.
Medicham: One of the best Choice Scarf users in the tier, and will be a better one after Tyrantrum gets banned. It threatens offense, and Life Orb sets threaten balance and stall a lot. Even though its bane is Spiritomb, teams not having Spiritomb risk getting dented by Medicham, as its other checks get hit by its other STAB or coverage. Furthermore, Medicham is seen being used with a Pokemon that can lure Spiritomb to be more effective. A good example is Colbur Berry Meloetta with Dazzling Gleam. On the down side, if Mega Steelix gets banned, we're going to see more of Fairy-types, especially Granbull and Aromatisse, which are good checks of Medicham. This means that even though Medicham will be more dangerous, it still has checks which will be more used. Medicham, for now, should stay in A rank.