Every one of my discs bring something to my game, or I would put them in a closet and never throw them.
But the most 'unexpected' one was the Leopard. I only got one because everyone kept talking about them. It was understable, but unlike the Archangel, Roadrunner, or Monarch, the Leopard is very predictable.
When I got one max weight, and found out I could release it on a hyzer, and it flew straight as an arrow, all my fear of playing wooded courses disappeared. In fact, I am very aggressive on tight courses now because of the Leopard and what I can do with it. It had the most unexpected impact on my game.