well there is none, that may sound cliche but it's the truth your pokemon's strength relies on it's trainer, it's nature and of course it's move set, so what if you have a strong pokemon but you don't know how train it well and does'nt know what moves are good with it, and what about meganium not knowing any good attack, well if you know meganium is my favorite grass pokemon starter, but i'm not saying this without proof, meganium can learn leaf storm which is a good grass-type move if you know how that is(i can teach how if you like)
the best move for grass-type pokemon is:solar beam or grass knot,leaf storm(but only use this if your sure to use it)energy ball and synthesis or alternate type move
well you see what i'm saying is that there is no weak pokemon, it all depends on how you train it