I would keep Razor Leaf on your Meganium. The high critical hit ratio is nice, 25% chance of getting a critical hit. Giga Drain is ok, but the low PP is kind of a downside IMO. Plus, it's base power is only 5 higher then Razor Leaf's, (55 for Razor Leaf, 60 for Giga Drain.) but with a regular critical hit ratio. Your moveset looks really good to me, if you are going to replace anything, I would replace Earthquake with Synthesis.