it depends on what you have
leaf storm is the best for mixed and special attackers
leaf blade or wood hameer for physical attackers
synthesis for a wall of some sort
overall i would say leafstorm is the best
I personally like leaf blade the best because there are no negative consequences to it (i.e. frenzy plant makes you recharge, wood hammer does recoil) and it has a high critical hit ratio (right?). Energy ball is pretty good too. Giga drain would be my favorite if it has just a little stronger.
I rarely use the Grass-Type moves.
but when I do, Leech Seed ALWAYS comes first.
Screw Leaf blade, Wood hammer and sh*t,
leech seed is the ultimate Grass-Type move <3
But seeing that Leech seed isn't here (h8ers!!)
I have to choose Giga Drain.
If you have high attack power you go with leaf blade if you have high special attack power you go with energy ball which is actually better than leaf blade.
Energy Ball's my favourite, due to its BP and its ability to be learnt by many Pokémon. I'm also a fan of Leaf Blade, although that's just due to the win that is the Treecko line.