Protoss vs Zerg

The 2 most important and valuable units in this match up are your high templars and your reavers. The Templar is the main one cause it attacks both air and ground with it's Psi-Storm ability. This attack kills all but the ultralisk in one storm. You can lay waist to an entire attack with 2-3 of these. Because Zerg units have such low HP, they die fast. Reavers do a lot damage and also do splash damage that can take down 2-4 surrounding units. I would suggest going reavers only if your opponent seems to be going mass ground units. Always have 1-2 templars at every base just in case of any attack. Templars are good to use when you don't know what your opponent is really going for or they are mixing ground and air units in their attacks. A reaver drop could hurt a Zerg player since they need a good economy to produce enough units to stop you.

Coursair's are going to be one of your best friends in this match up. If you get enough of these (4-6) without your opponent seeing them, you can go on an overlord raid and get a lot of kills since he won't know about them yet. Always keep a few of these guys handy for d-web, chasing of mutalisks, or chasing away the Zerg detectors so your dark templars can reek some havoc. D-web those pesky sunken colonies and move goons in to range without getting hurt. You can d-web lurkers until new detection comes.