Ensabanur is pretty much right on with what you want to do for the most part, however there are even more options, I'll detail a few:
Against m&m:
If it's more 'rauder heavy, zealots and sentries work fantastically early on as forcefield allows zealots to get in range and do damage.
Void rays are another option to delay the Terran push and keep him in his base while you build up a force/expand
Immortals are fantastic when the army sizes are small, they lost a lot of power against large infantry balls, however, as they melt so fast even with the shield.
Collosi are always a good choice against heavy infantry, along with good forcefield use, they completely destroy any kind of marine/'rauder army, until vikings come into the mix.
Templar + Chargelots are yet another fantastic way of dealing with m&m balls, storm absolutely demolishes marines and softens up marauders for the zealots to kill.
As you can see, there are many, many ways to deal with just m&m, however, when they add the additional units (ghost/viking/medvac/tank) you have to mix it up a little.
Ghosts: Immortals fall extremely fast to emp + 'rauders, collosi are fine as long as they're kept back and your sentries don't eat an emp. chargelot/templar also do fine as long as you keep your templar spread out to avoid the emp (can also feedback ghosts).
Vikings: They shut down collosi play unless you want to invest a lot into stalkers (with blink) to protect the collosi.
Medvacs: Chargelot/templar is especially good at dealing with this for a few reasons. Feedback is very useful against the medvacs and because you have an only warp-gate army, you can deal with drops by warping in templar/zealots to deal with it.
Siege Tanks: Can be dealt with my abusing their immobility (by taking additional expansions and engaging while they're on the move). Void rays are great if they're light on marines, and phoenixes are also a good choice for when you want to be agressive as you can lift the sieged tanks and run your army in to clean up the now tankless terran (with phoenixes I generally just go sentry/stalker/zealot with twilight upgrades if I can affoard it by then with an eventual transition to high templar).
Hopefully that should give you a few ideas, and by no means is this list exhaustive.
Author: Statement
Against m&m:
If it's more 'rauder heavy, zealots and sentries work fantastically early on as forcefield allows zealots to get in range and do damage.
Void rays are another option to delay the Terran push and keep him in his base while you build up a force/expand
Immortals are fantastic when the army sizes are small, they lost a lot of power against large infantry balls, however, as they melt so fast even with the shield.
Collosi are always a good choice against heavy infantry, along with good forcefield use, they completely destroy any kind of marine/'rauder army, until vikings come into the mix.
Templar + Chargelots are yet another fantastic way of dealing with m&m balls, storm absolutely demolishes marines and softens up marauders for the zealots to kill.
As you can see, there are many, many ways to deal with just m&m, however, when they add the additional units (ghost/viking/medvac/tank) you have to mix it up a little.
Ghosts: Immortals fall extremely fast to emp + 'rauders, collosi are fine as long as they're kept back and your sentries don't eat an emp. chargelot/templar also do fine as long as you keep your templar spread out to avoid the emp (can also feedback ghosts).
Vikings: They shut down collosi play unless you want to invest a lot into stalkers (with blink) to protect the collosi.
Medvacs: Chargelot/templar is especially good at dealing with this for a few reasons. Feedback is very useful against the medvacs and because you have an only warp-gate army, you can deal with drops by warping in templar/zealots to deal with it.
Siege Tanks: Can be dealt with my abusing their immobility (by taking additional expansions and engaging while they're on the move). Void rays are great if they're light on marines, and phoenixes are also a good choice for when you want to be agressive as you can lift the sieged tanks and run your army in to clean up the now tankless terran (with phoenixes I generally just go sentry/stalker/zealot with twilight upgrades if I can affoard it by then with an eventual transition to high templar).
Hopefully that should give you a few ideas, and by no means is this list exhaustive.
Author: Statement