#37 Game Preserve (Mercadian Masques) Why is it bad
This unassuming rodent, printed in Return to Ravnica, basically required an immediate answer, or it would multiply out of control, each copy making the rat horde, and the rats themselves, grow bigger and bigger
Whatever you exile has to be a whole lot scarier than the Incubater token for you to remove it for 3 mana at instant speed
(Mordor Guard (7C287) deserves special mention, since he doesn't remove threats if he's placed on Web.) Bear in mind that the the Moria swarm engine and Goblin Hordes (11R123) don't work when the minions don't go to the discard pile normally
You can untap a mana creature or mana rock that makes more mana and then either have a free spell or more
Black generally doesn't have very many ways to untap creatures, but you can still work around this to get multiple casts in a turn with cards like Thousand-Year Elixer