The issue with burst spells is that you effectively discard a card to gain some life, which doesnt progress your board state
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While this would already be quite devastating, whenever a creature with a slime counter on it dies, Toxrill's controller creates a 1/1 Slug token, converting opponents' creatures into a Slug army
You also need to consider power level when thinking about the best cardsMana Drain is one of MTG's strongest counterspells but its not an appropriate card for every cube
Lets compare their value: While Fervor is cheaper and on a noncreature permanent (thus making it harder to remove), Anger provides a 2/2 body that wants to be removed