Here's a simple truth of spellbook design:
A well-rounded spellbook will and should defeat a myopic one every time. There are so many ways that you can "change" the flow of this game that I might just be inclined to argue that nearly every card is a "game-changing" card. It all depends on the circumstances.
Here's a few:
Teleport: The ability to throw something wherever you want it is incredibly strong. The ability to do it as a quick action is devastating.
Dispel: The ability to tear down enchantments is a cornerstone of balancing enchantments. When played on the right enchantments at the right time, Dispel is easily in the top 3 for the "game-changer" category.
Cheetah Speed: Giving Fast to your Mage or one of your creatures can completely throw off another player in terms of positioning, or surprise an opponent at an opportune time. It can be used to facilitate your Mage's escape from a bad spot, or it can be used aggressively. It is similar to Teleport in these regards.
Poisoned Blood: As someone else pointed out, it can seal the deal in games during the late game. Most Mages will include at least a few "Oh SHIT!" Healing Spells in their spellbooks, and if you drop this on a Mage when you're close to victory, they have to waste a precious action on Dispelling it (if they have one left) or be doomed. Either way, it's a strong play and can definitely change games, if not outright win them.
Seeking Dispel: While it's a shot in the dark, Seeking Dispel is incredibly cheap, both for spellbook inclusion and for casting. It can cheaply pick away carefully laid defenses or traps. The obvious fear is that you might hit a Decoy, but it's a risk worth taking more often than not.
Any quick action attack spell that hits 2 or more squares away and has the opportunity to Stun, Daze, or Push an enemy. Timing is everything, and if you pull of a lucky shot on one of these it can DECIMATE a player's carefully laid plans for an entire turn.
Reverse Magic: It's expensive, but it can ruin the day for your opponent if they throw a particularly bad spell your way. It can change games.
And the list goes on ad infinitum. There are just SO many ways to "change" the game that it's almost easier to pick out the spells that are not game-changers in one way or another...
-nihil
