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.