The best moveset for Meganium in the current competitive meta typically features the moves Solar Beam, Dazzling Gleam, Protect, Weather Ball.
In the Pokémon Champions meta, Meganium can be countered by exploiting its weaknesses (bug 2x, fire 2x, flying 2x, ice 2x, poison 2x) and using effective check/counter teams.
The best item for Meganium is typically Meganiumite, based on high-level tournament and ladder stats.