Venusaur

Mega Venusaur's great physical and special bulk, key resistances to prominent types like Water, Electric, Grass, and Fairy, and ability Thick Fat to take away two weaknesses render it a great defensive check to a number of the best Pokemon in the tier. More specifically, it is able to switch into and beat almost every variant of Mega Mawile, Tapu Bulu, and Magearna and check Kartana and Tapu Koko very handily. It can also take advantage of a lot of more passive defensive Pokemon, such as Ferrothorn, Tapu Fini, and Tangrowth. Mega Venusaur's Poison typing allows it to remove Toxic Spikes on entry, making it a valuable addition to teams weak to them. It is able to differentiate itself from Amoonguss with Thick Fat, which enables it to take moves like Ice Beam from Magearna and Fire Fang from Mega Mawile comfortably, much bigger immediate bulk, and significantly better offensive presence. However, Amoonguss has access to Regenerator, can hold an item, and doesn't take up the team's Mega slot, giving it a decent niche over Mega Venusaur. While it is a good Water-type answer, it is not a reliable Choice Specs Greninja switch-in on its own, as Dark Pulse does too much for Mega Venusaur to check it consistently; thus, it requires a different one besides itself. While Mega Venusaur's typing is fantastic defensively, wallbreakers that can exploit said typing like Tornadus-T, Mega Alakazam, and Tapu Lele pose a huge threat to it. Furthermore, its main form of reliable recovery, Synthesis, has a measly 8 PP, meaning Mega Venusaur cannot last deep into late-game without outside support, especially since it can be forced into awkward situations where it is forced to use Synthesis more often than it'd like. Its recovery is also lowered to 25% by both rain and sand, reducing its otherwise high effectiveness against those archetypes.