Thassa, God of the Sea is a fine choice
It defined an entire victory condition - milling. Not too powerful, but it was epic for its time, if for no other reason than it got folks thinking about other ways to win the game. - Radagast Millstone more-or-less single-handedly invented a win condition in Magic, and created a deck archetype (and a word!) that extends to basically every card game now in existence
There are two boards Red & Blue, with the ability to capture Pokemon by hitting the pop bumpers six times in a two-minute period
Game Changers are prohibited in Brackets One and Two, and unrestricted in Brackets Four and Five
Those looking to grab one can find it on Amazon, or at Target, Barnes & Noble, Walmart, and card shops
You might even find the best P okmon packs to buy for rare cards this way