Best counter for hydras:
Terran: Mass marines with stim packs & medics. Hydras own marines without medics, but M&M is, no doubt, the best counter terran has for hydras. Golls vs zerg wont work, because zerg will take the entire map and metal is not mobile enough.
Protoss: Zealots + goons + high templars. PSI storm owns hydras. Reavers are good too, but i find templars more cost efficient in mid game. In late game, dropped reavers can be usefull combined with corsairs and templars.
Zerg: The only way to beat a good zerg player is mutas + zerglings. If he is making hydras, just kill him. Hydras do 50% damage to mutas BTW.