A few new grass type moves have been introduced and some older moves have been increased in power in the 4th generation... So with so many powerful and effective moves around which is the best considering power reliability and effect?
I really like Energy Ball, after playing three generations without having a reliable Grass move to use which a lot of Pokemon can learn, fourth generation gave us what was needed. I know that Grass Knot (which isn't in the poll) is preferred in some situations, but my choice has almost always been Energy Ball when it comes to a Special Grass move. Frenzy Plant just sucks and Giga Drain is usually inferior to Energy Ball/Grass Knot. Leaf Storm can be useful if backed up by STAB, high SAtt/Spd and a Choice Specs, otherwise probably not.
As for the physical side, Seed Bomb (not in the poll either) is probably the best as alot of Pokemon are able to learn it due to the fact that it became a Move Tutor move in Platinum and HG/SS. Leaf Blade and Power Whip are not bad, but there are not many Pokemon which learns them compared to Seed Bomb. I am not sure about Wood Hammer though, it can score high damage but I don't know if the recoil is worth it.
My vote goes to Giga Drain, whith Synthesis on a second spot. Giga Drain made all frail li'l grass mon's very annoying. In fact, not only grass mons could teach it. Crobat, Gengar, Uxie, Yanmega, etc, all could know it, which makes it a handy tool. In 5th gen it's power even got a 15+ boost. In 4th gen it's still is a move to count in.
Synthesis also made a revoulution for grass types. Now they were able to heal whithout Mega Drain/Leech Seed/whatever they used. It also made up some cool strategies like putting a Tangrowth on a sun set. Chlorophyl, Solarbeam and Synthesis made it a great sweeper and tank. Shame that it didn't work like Recover though. It would be above GD then.