I would go for Feraligatr. Meganium is more a supporter, which isn't necessary in a game of hyper offense. Typhlosion works, but its movepool is quite shallow. Feraligatr can also learn various HMs that the other two cannot. For these reasons, I would say Feraligatr. Here's a moveset:
Feraligatr @ Mystic Water
This allows you to hit for tremendous neutral and super effective coverage. Feraligatr is a bit slow, but I still believe it's the best option. If you must use one of the other two, don't go for Meganium. There are far better Grass types. However, as Typhlosion is almost as good as Feraligatr, here's a set for it:
Typhlosion @ Charcoal
-Swift / Focus Blast
Typhlosion really doesn't have a good movepool, it's a bit trickier than Feraligatr to use.
Hope I helped!