Venusaur

Why is this Venusaur Calm? Is this viable?

Yes! More often than not, Venusaur will be facing damage from Special Attacks, such as Flamethrower, rather than Physical Attacks.

As a Mega-Venusaur, the base Special Defense stat goes up 20, bringing the effective stat to 372.

With the Thick Fat ability, If a Pokemon uses a Fire- or Ice-type attack against this Pokemon, that Pokemon's offensive stat is halved when calculating the damage to this Pokemon. When used in Rain, it is even more broken.

This makes Mega-Venusaur with the Calm ability and the moveset of Giga Drain / Synthesis, not only a sweeper but also a wall.