Sp. Atk: 83
Sp. Def: 100
Overgrow: Increases the power of Grass moves by 50% when at 1/3 health or lower.
Leaf Guard (DW only): Prevents status from afflicting it in Sunny weather. Does not block self-induced status aside from Rest.
Tackle, Growl, Razor Leaf, Poisonpowder, Synthesis, Reflect, Magical Leaf, Natural Gift, Petal Dance, Sweet Scent, Light Screen, Body Slam, Aromatherapy, Solarbeam
Toxic, Hidden Power, Sunny Day, Hyper Beam, Protect, Safeguard, Frustration, Earthquake, Return, Double Team, Facade, Rest, Attract, Canon, Echo Voice, Energy Ball, Giga Impact, Flash, Swords Dance, Level Ground, Dragon Tail, Grass Knot, Swagger, Substitute, Cut, Rock Smash, Strength
Ancientpower, Counter, Flail, Grasswhistle, Healing Beam, Ingrain, Leaf Storm, Leech Seed, Nature Power, Refresh, Vine Whip, Wring Out
Grass Oath, Frenzy Plant
4th Gen only:
Fury Cutter, Headbutt, Mud-Slap, Outrage, Seed Bomb, Role Play, Snore, Worry Seed, Roar, Bullet Seed, Giga Drain, Iron Tail, Secret Power, Captivate, Sleep Talk, Rock Climb
In comparison to his fellow GSC brethren, Meganium is definitely a mixed bag. Relying on defensive powers instead of muscling his way through, Meganium faced stiff competition from similiar Grass starter Venusaur due to the latter's immunity to Poison, neutrality to Bug and Poison, as well as a higher Sp. Attk stat and access to Power Whip and Sleep Powder.
In Gen V, the distinction between the two have become more pronounced. Venasaur got Chlorophyll as well as having receiving a boon in the boost to Growth, making it much more deadly.
Meganium on the other hand, while receiving little more moves to add to its already defensive movepool, got a solid ability in Leaf Guard, finally allowing him to bask in the glory of status immunity aside from Substitute.
Unfortunately? Getting Leaf Guard has some heavy costs. Since it's likely that Chikorita from DW will be Male, in keeping the trend with the other starters released so far, it means Meganium loses out on a lot of very useful moves, such as Leech Seed, Seed Bomb, Leaf Storm and Flail, greatly reducing its offensive potential and meaning it can't pull off Subseeding anymore.
Still, Meganium got other perks. The new move Dragon Tail is great for him, since it can now phaze off opponents with fairly good defensive stats. His natural support movepool is still great, with Screens, Aromatherapy, Synthesis and Body Slam, and got a boost in Petal Dance's power. Those not running Leaf Guard also get access to the boosted Giga Drain.
Overall, Meganium is still a decent choice for anyone needing support. A great support movepool, passable offenses and a decent speed rating for a cleric allows Meganium to stand its ground, even if it has a lot of obstacles in its way, including a poorish defensive typing and many movepool situations. It might have disadvantages compared to Uxie, Cresselia, Blissey and the like, but it still has its own standouts over each of them individually.
Generally, despite Leaf Guard being a good ability, Meganium should generally be running Overgrow, as it gains a lot of movepool options by doing so.
252HP, 252Def/SDef, 4SDef/Def
~Dragon Tail/Body Slam/Leaf Storm/Giga Drain
Meganium has good 83/100/100 defenses and access to Screens, which make it a decent choice as a Dual Screener. Leech Seed further augments its staying power while sapping opponents of their health. Leaf Storm and Seed Bomb are its options for an offensive move, Leaf Storm has more power, and is great for Meganiums who come in, set up Screens, hit one or two Pokemon with Leech Seed and hit them hard before switching out. Giga Drain has a good new BP, allows Meganium to keep alive even longer, and has some synergy with Overgrow should you ever need it. Body Slam is a great option, and allows Meganium to support your team further by potentially paralyzing potential threats and likely racking up a bit more residual damage as well. Dragon Tail might seem counterproductive with Leech Seed, but it allows Meganium to make a decent shuffler for Spikes-based teams, as Leech Seed can force a few switches and Dragon Tail swaps your opponent's team around, resulting in quite a bit of residual damage. Adjust the nature accordingly, depending on which stat you want to boost (Bold/Impish for Def, Calm/Careful for SDef), and which attack move you're running (Bold/Calm for Leaf Storm/Giga Drain, Impish/Careful for Dragon Tail/Body Slam).
One should be wary of Tentacruel if not running Dragon Tail, as the jellyfish can pack Liquid Ooze to make things annoying. Meganium can run Earthquake, but it's generally a very unorthodox option.
A Leaf Guard set is possible for those playing a Sun-Stall team, but a few modifications have to be made. Meganium can't run Leech Seed, so will likely be running Toxic for damage. The fourth move slot can be Dragon Tail, which is more attractive now that you don't have to worry about Leech Seed and now you can spread Toxic around if you don't have Toxic Spikes up. Synthesis is also an option, and works great for those using Meganium in a Sunny Day team already. Petal Dance is Meganium's best remaining offense, but is a poor choice on something not meant to be a sweeper in the first place; Energy Ball would likely be the better choice. Body Slam is a less attractive choice if you're running Toxic, too.
248HP,128Def, 128SDef, 4Spd
~Leech Seed/Dragon Tail
Meganium is an extremely sturdy cleric, and is one of the few with access to a recovery move as well. The idea is still pretty simple; switch Meganium in when you need to, Aromatherapy, recover health if you need to. You can then attempt to parahax, or Seed and run or phaze a little.
The EVs here are for balanced defenses, but one can easily alter them to create a more specialised defensive set if one wishes. 248HP allows maximum Leftovers recovery.
Once again, a Leaf Guard set is possible, just noting that Leech Seed can't be used.
252Att, 252Spd, 4HP
An old and rarely used set from the 4th gen. This set definitely requires Overgrow, as it greatly boosts Overgrow and the main moves on this set are 4th gen only anyway.
One basically Subs and Dances till Salac range, then sweep with a 200BP Flail and a 180BP Seed Bomb (after Overgrow and STAB). If one is very confident in Meganium's ability to take hits and think they can predict well enough to get Meganium into Salac range without Substitute, Earthquake can be used for more coverage, but is very risky.
The increased prevalence of priority and weather definitely hurts this set a lot. Meganium's Attack set isn't really that great either, though it has good BP for its moves and its stats and typing allow it to take weak priority, especially Aqua Jet.
252HP, 128SDef, 128Def
An unorthodox set, but can be very annoying to take down and can be very frustrating in the hands of a SpikeStacking team. Dragon Tail bounces opponents around, Aromatherapy allows Meganium to act as a cleric, but the main synergy lies with Sleep Talk. Sleep Talk has a 2/3 chance of letting Meganium wake up instantly, allowing it to recover yet again or phaze the opponents with normal priority. With some luck Meganium can last a long time, and opponents can't even set up on it at the risk of being forced out by Dragon Tail at any time.
Overgrow is a must for this set, since Sleep Talk is 4th gen only.