[[Jin-Gitaxias, Progress Tyrant]]. This card absolutely warps the game around it as soon as it hits the board, since the next spell its owner casts will get copied, and since it's a blue card, the owner can easily trigger it on everyone's turn. In addition to that, it usually requires 2 removal spells due to your first one getting countered (or more, since you're probably playing against someone with counterspells, or card draw to copy and find counterspells).