For ZvT it's usually defilers. T will counter with vessels, but Plague will suffice to counter it.
ZvZ, again, rarely do ZvZ go hive play, so not much. ZvZ is mutaling, so Plague can again be good with enemy Muta stacks.
ZvP it's Ultraling. Defilers won't do jackshit because of the Archons and the Zealots dealing splash and melee damage, respectively. Also, Maelstrom is a good counter for Ultraling.
Basically, Cracklings should be roughly a third of your army. Whether it is Ultraling or Mutaling, they do insane DPS and distract the enemy. They also mostly take the bulk of the damage.
Lol why do I know this I'm not even Zerg.