Venusaur

Mega Venusaur succeeds primarily thanks to its positive matchups against many very common Pokemon, including Tapu Fini, Tapu Koko, Kartana, Diancie, and Zapdos. With its bulk, typing, and ability, it's difficult for many teams to break, and a naturally decent Special Attack stat means that it can still hit hard enough in most scenarios. In addition, Venusaur can tweak its moveset to fit its teammates' needs, but its inability to run all of its solid options leaves it lacking in at least one area that it could otherwise excel in. Moreover, a middling Speed tier means Venusaur often needs to take hits before attacking, so it risks being taken out if it doesn't focus on remaining healthy. Speed control and Chlorophyll help ease this a bit, but neither is permanent and the latter is only available before Mega Evolution.