Celebi EX
Up until the Mythical Island expansion, Grass was basically ruled by Venusaur EX and its bulky-offense play-style. This made Grass a great type, but there were times when it was a bit lacking in raw power. And then along came Celebi EX, and its ability to throw out nuclear coin-tosses!
Celebi EX is a very interesting card, to say the least. It's heavily RNG based, but that RNG can be quickly turned in its favor with additional energy. This makes Celebi EX extremely energy-hungry, but the dividends paid out for the investment can be downright staggering, as with enough energy it's possible for it to semi-reliably take down literally any card in the game in a single hit. This is a card that you are going to kind of want to build around, but it also can't hurt to just throw one into a Grass deck focused on another strategy as an alternate win-condition.
Celebi EX + SerperiorGo to Deck
Celebi EX thrives on energy, so getting Lilligant out early to generate energy, and then having Serperior on the bench (you only need one, they don't stack) before Celebi EX hits the field can easily result in 10+ coin flips in just a few turns, which is more than enough to KO literally anything in the game more than 50% of the time on average.