Meganium
Type Tier
Grass BL
Statistics
Stat
HP
80
 363
Atk
82
 262
Def
100
 298
SpA
83
 264
SpD
100
 298
Spe
80
 258
Name Item

Staller

 Leftovers
Moveset
~ Razor Leaf
~ Reflect / Light Screen
~ Leech Seed
~ Synthesis

Meganium is one of the best underused Pokémon. Leech Seed whatever comes in, use Reflect or Light Screen depending on the opponent, and wear it away with Razor Leaf. You can also ditch Leech Seed and use both Reflect and Light Screen to set up a sweeper.

Name Item

Swords Dancer

 Leftovers
Moveset
~ Body Slam
~ Swords Dance
~ Synthesis / Giga Drain
~ Razor Leaf / Earthquake

Though it doesn't get any physical STAB, Meganium can use Swords Dance well in the underused metagame with those nice defenses. Body Slam will paralyze stuff, and Razor Leaf will hit Ground-types. Or, if you want, you can use Giga Drain to attack AND recover, and Earthquake for Fire and Poison-types.

Other Options

Meganium's defenses can pull off Rest, but I don't recommend it. Also, Counter works great with its Defense and HP (even more effective with Light Screen). Watch out Skarmory, Heracross, and Zapdos! Meganium can also learn Ancientpower for coverage on the Swords Dance set, but there is no immediate use for it bar hitting Zapdos.

Opinion

Meganium is a very versatile Grass-type that can attack and support well (it's better at supporting). It's one of the best underused Pokémon and works well in the standard metagame as well. It doesn't learn Sleep Powder, or Stun Spore, which is a bummer, but otherwise this is definitely something to consider.

Counters

Moltres, Charizard, Houndoom, Skarmory, Heracross, Zapdos, Articuno, Muk, Tentacruel, Blissey, Victreebel, Jynx, Forretress, Misdreavus, and Celebi are all viable counters. The first three are Fire-types who Meganium can't do anything against except Ancientpower (which you never see on Meganium anyway.) All the others also sport resistance to whatever Meganium might throw at it, and in most of these cases they will always run an attack that hits Meganium for super effective damage.

With the staller set, it must also be noted that Meganium is going to have a hard time taking down Vileplume and Bellossom as well as other Grass-types due to their resisting Razor Leaf and immunity to Leech Seed.