For the most part Energy Ball because its learned by a surprisingly wide variety of Pokemon. Leaf Blade in close second, simply because its not really learned by too many Pokemon. Sceptile, Gallade, and Virizion (5th Gen) are the only ones really worth mentioning..
Energy Ball FTW! BTW, does anyone find it strange that energy ball can be learned by many psychic types as well? Plus nobody would would ever would have thought that it is a grass move by name alone until they checked the move screen. It is literally a copy, edit, then paste version of shadow ball. PS: I love petal dance.