PvT (main units)
-zerglings: high temp storms are very good vs zerglings, killing huge amounts of them in seconds, just make sure to keep them safe in the back. once HT are out of energy, morph into archons, they deal splash and are good vs lings too. Collosus are also great, with great splash damage, but again keep them in the back. Zealots can be good vs zerglings, especially at choke points, in the open the lings will have the advantage.
-hydras: Always good to have sentries in your army, same it true vs hydra because of guardian shield. HT's are also good with storm as well as zealots cause of their high dps.
-banelings: Ht storms and archons are good, as well as immortals because their of their shields.
-roaches: obviously immortals, they are so good vs roaches. Roaches have high damage but slow attack, meaning that their attacks become ineffective vs the immortals and get taken out in 3 shots.
-mutalisks: phoenix is a good counter, but likely you won't have a big army of them when u get attacked, so stalkers should hold them off along with some archons for splash.
-ultralisks: any air like voids, carriers, and motherships do well obviously. On the ground immortals do well because of their shields, but keep them spread for max efficiency. Stalkers deal out pretty good damage, as well as collosus if there are multiple ultras, but be careful because if you are too grouped up they will tear through you.
-brood lords: best counter is air vs them, void rays especially. Blink stalkers should only engage them if you can snipe them out fast.