It turns all nonbasic lands into mountains, which completely shuts down decks that rely on fetches, shocks, or utility lands
Does Exile Work on Indestructible
Pound for pound, one mana for a 2/1 creature with no drawbacks can't be beat
Honorable Mentions This card is like a renewable 2 power creature that is very hard to kill
Man-o'-War (Mirage/Visions/Weatherlight) Undo and Man-o'-War were often seen together in packs, and players debated which was better: the sorcery which took out two creatures, or the 2/2 creature which took out one
MANA DRAIN Counterspells are dime-a-dozen in blue, so while they are in many ways replacement level, there are some that stand above the rest and make it into this list