Ok, first off i voted lurker, because hydra/lurk is my favourite zerg strategy and lurker defence is one of the coolest ums maps i have ever played
But anyway, there is a meaning of every unit in the game, and since obviously some of u cant see it, i will try to explain.
Corsair:
The corsair is one of the best units in the game, and it has opened for many new strategies. Corsairs do splash damage, so 4 corsairs will own 12 mutas or 12 wraiths if they are stacked together.
A very popular strategy is to get a few corsairs in early game and start hunting overlords to cut of enemy suply and waste their minerals on making new ovis.
On island maps, the most used toss strategy is to do a corsair/reaver drop combo. This gives u the best anti-ground and the best anti-air units of the protoss combined.
Medic:
Regular SC was very unbalanced and zerg totally owned terran by massing hydras. In BW, lurkers and medics have totally balanced that MU. Medics + marines counter hydras and irradiate/tank support counter lurkers, while lurkers counter marines and hydras counter tanks. Its all about unit controll.
Thanks to the medic, it is now possible to do a M&M rush vs toss in early game if he doesn't tech to reavers/templars.
Valkyrie:
Valkyries do not deal a lot of damage, but they attack many units at the same time. They are used for backup, they are not intended for massing. Valkyries counter mutas, guardians, they can be used together with BCs to counter only BCs, and they totally own wraiths.
Devourer:
The devourer attack is one of the best in the game. First of all, it highers the cooldown of the enemy unit, making it attack slower. Second, the acid spores makes the unit take more damage from each attacking unit. 9 acid spores = 9 more damage taken from each hit.
Devourers is the only counter toss has for corsairs (corsairs own scourges) and is also good vs wraiths, BCs, valkyries and scouts.
Lurker:
The lurkers are one of the most powerfull units in the game. Against terran they are used to counter infantry. 8 lurkers can take out 30 marines if the marines aren't controlled properly. A lurker + hydra drop in the terrans main base while the enemy is attacking can take out the entire base and ruin his unit production.
Against toss the lurkers must be used against lots and goons. Psionic storm can be devostating for a zerg army, but a good zerg player will be able to controll his hydras so they wont get killed. After the lurkers came, the zerg player is finally able to contain a toss player in early game, and that gives him time to expand.
Dark archon:
Not many people see the potential of the dark archon. Maelstorm can freeze hydras so they cant be moved away from a psionic storm, and something most players havent seen is, it can stop ultras + lings in swarm!
Mind controll is rarely used, but if u are attacked by carriers or BCs it can be usefull. Mind controlling a drone or SCV takes too long so its not very effective.
Feedback kills a high templar, defiler or science vessel in one cast. It also removes a great deal of hit points from BCs.
Dark archons are not very cost effective in early game, but in late game they can be really good. They are the most effective in ffa's, because u can be attacked at any time and DAs are very good at holding back enemy attacks.
Dark templar:
Dark templars deal a lot of damage, so together with zealots and goons they can be used to push back tank pushes. A DT rush is rarely used because it can be scouted, but a hidden DT rush can catch a terran of guard. Unexpected DT drops can also be hard to counter.
