Who says you can't have duplicates? I'd totally pick Call the Warpath myself. XD
If we can't have duplicates, however, I'd have to go with Hidden by Clouds. Sure one can be dealt with by removal, but if you have three, you can basically have the first one pull out a second, and then the second pulls out the third. Now you need 3 answers in order to stop the rush of beasts.
(or one answer to rule them all. I'm looking at you, Calamity and Oblivion.