Personally, I'd say Bellossom of all things outclasses Meganium these days in-game. It might not be quite as bulky, but it can learn Sleep Powder in its unevolved forms and Leaf Blade via Move Relearner, while Meganium is stuck with a choice between Razor Leaf, Magical Leaf, and Petal Dance for STAB damage until you reach Kanto. Meganium's poor STAB options are really disappointing nowadays, when other Grass types can get actual damaging moves.
Looking at other options, Vileplume and Victreebel can get Sleep Powder and STAB on Sludge Bomb at least, and Victreebel gets Leaf Blade eventually as well. Exeggutor gets Slee Powder and Leech Seed by level as an Exeggcute, and Wood Hammer at 37 or Seed Bomb by relearner if you dislike recoil.
Problem is, Typhlosion users will pick themselves up a red Gyarados/other water type, Feraligatr users already have a water type, and there is nothing notable in game that you need Grass > Water to warrant even bothering with any grass type(obviously, other than starting with Meganium).
So the IVs reset everytime you faint the legends? Does this mean I can save --> capture Raikou, then if I don't like its IVs, I can kill it and they reset? Or does this not work for roamers? Either way, I will be playing through multiple times to get both a Timid and Bold Suicune, along with the usual Bold Lugia, Adamant Ho-oh, etc.