Best grass type direct attacks:
Leaf Blade > Giga Drain > Solarbeam > Razor Leaf > Mega Drain > Absorb
Reasons:
Leaf Blade has more damage than Giga Drain, and a high critical hit ratio.
Giga Drain is direct damage, and heals half the damage inflicted.
Solarbeam needs some setting up/charging time (Sunny Day), which is why despite its power is not a higher priority.
Razor Leaf is less powerful than Giga Drain, more powerful than Mega Drain, and again has a high critical ratio.
Mega Drain is half the power of Giga Drain, and Absorb is half the power of Mega Drain.