Meganium

Through both Great and Ultra Leagues, Meganium's greatest competition is its fellow Grass-type starter Venusaur. Meganium's niche is that it doesn't take super-effective damage from Psychic, and can handle Steel-types more effectively thanks to Earthquake. However, taking super-effective damage from Venusaur, combined with Poison's often more relevant coverage, on the whole makes Venusaur the stronger choice.

Great League: 3.5 / 5

Meganium positions itself in Great League as a bulky Grass type that takes full advantage of its access to Frenzy Plant. While being a pure Grass type has its pros and cons, Meganium overall is an excellent pick and can hold its own in many fights. Access to Earthquake keeps Fire and Poison counterpicks cautious, and also allows Meganium to wall most Electric types depending on shield usage. However, Earthquake's niche against common Pokémon like Probopass and Bastiodon doesn't do it enough to make this weird dinosaur a better pick than Venusaur in most cases.

Ultra League: 3 / 5

Meganium's stat distribution suits it well for Ultra League, though capping at 2410 CP means that it'll need to powered up to level 40 to be serviceable. Just like in Great League, Meganium excels in its Grass type role and can even win against a Lapras with Ice Shard. It's no match for Giratina though.

Master League: 1.5 / 5

Meganium's maximum CP of 2410 handicaps it from achieving top ranks in Master League, though Frenzy Plant still allows it to threaten Pokemon weak to Grass like Groudon and Kyogre. Giratina still top dawg. That said, it's probably better to look elsewhere if you're trying to handle the top threats. Just because it can perform, doesn't mean it's your best choice.