Protoss air-to-ground force suck...period.
Their air comes too late, cost too much and does too little to ground troop.
Going air means losing Templar or Reaver Tech...both are EXTREMELY efficient at taking out masses of ground troops...and in case of the templar...air also.
Tell yourself, in pro matches (WCG, CPL...etc) have you seen Toss V Toss matches where one side goes air?
Nope.
Notable exception : Corsair
This unit changed the way Protoss played.
Toss v Zerg, Corsair is the ULTIMATE air threat.
Scout has always been and will probably remain useless.
Corsair owns mutalisk...
Corsair moves freaking fast and has splash...great for scouting and taking out overlord.
Finally...DT + Corsair expand Vs Zerg anyone?
Who has the best non-casting air?
Lets see about Terran...
Valkyrie : Goliath is 100000 times better for anti-air purposes if you're going to get an armoury.
Wraith : The only useful unit for Terran. Excellent at harassing Zerg and in Terran V Terran but otherwise hardly used. Anti-air department belongs to the Goliath.
Battlecrusier : Suxor~ !
So the answer is...ZERG!
Mutalisk : Excellent speed for scouting. Great ground attack for hit and run and even direct battles.
Guardian : Roxor!
Scourge : Great anti-air.
Devourer : Not much used but when combined with Mutas and Scourges, owns air hard. Creates a "stopwatch effect" in Zerg v Zerg matches where the mutaling player who gets 1-2 Devourer will probably win.