Zerg Vs Zerg

more straightforward and self explanatory, and you can usually counter each unit with more of the same but upgraded...

-zerglings: get more of them and upgrade, if not, roaches are quite good, plus their 1 armor makes them really robust. Later on, ultralisks, brood lords, and mutalisks will eat them up.
-roaches: Hydras are pretty good since they out-range roaches and have very high dps. Ultralisks will tear through them (like any ground unit really) and mutalisks obviously since they are air.
-hydras: a large zerglings/banelings force will decimate hydras. Anything else? Ultralisks..enough said.
-banelings: anything armored like roaches and ultras. Mutas and BL's too, but make sure to keep lings and hydras away.
-Ultralisks: Nothing on the ground is very good vs ultras, but if you have to, make sure to keep your units spread. The safe way is to use mutas and BL's to at least soften them up so you can take them out before they wreak havoc.
-brood lords: corrupters obviously good, but mutas too. I dont recommend hydras since they are outranged and can be taken out easily by BL's attacks, but if you must, snipe them as fast as possible.