This article will explain the Starcraft 2 units and counters so that you can familiarize yourself with it and then successfully counter any enemy unit that even considers attacking.
Zealot – Hellions, Colossi, Banelings, or Roaches
Stalker – Immortals, Marauders, Zerglings
Sentry – Hellions, Stalkers, Reapers, and Hydralisks.
Observer – Any stealth revealing unit or building.
Immortal – Any fast firing units
Warp Prism – Air attackers
Phoenix – Battlecruiser, Carrier, Corruptor
Void Ray – Viking, Phoenix, Corruptor
High Templar – Ghost, Zealot, Roach
Dark Templar – Biggest weakness getting revealed from cloak
Colossus – Ultralisk, Thor and Immortals
Archon Carrier – Thor, Ultralisk, Ghost (it should be noted that Arhcon is the only unit that receives normal damage from all other units)
Interceptor – Viking, Void Ray, Corruptor
Mothership – It’s slow which makes it more prone to EMP’s and feedback, and is also not so strong versus other massive units.
Hydralisk - Large Armored units, MMM, AOE spells, Brood Lords, Colossus’s, Ultralisks, Thor’s, Templars, Banelings
Queen – Off Creep, large groups of units, Roaches, Marauders, Marines, Hydralisks, Zealots
Zergling – Large armored units, balls of ranged units, blockades/chokes, Roaches, Marauders/Marines, Hydralisks, Zealots
Read some great Starcraft 2 Tactics.
Overlord – Anti-Air (especially when speed isn’t upgraded)
Infested - Overall Weak Support Unit, Roaches, Stalkers, Marauders, Marines
Roach – Marauders, Stalkers, Air, Well Defended Positions (short range), Colossus, Templar, Banshees, MMM
Overseer – Fast moving anti-air
Baneling – High HP armored units, Air, Ranged Balls, Roaches, Marauders, Stalkers, Large Units
Infestor – If left alone, micro, Air, Air to Ground, Tanks, High Templar
Brood Lord – Fast Anti-Air units, some spellcasting, Any Air to Air Unit
Ultralisk – Anti-Armor Units, Air units, closed terrain, high damage units, Banshees, Thor’s, BattleCruisers, Void Rays, Carriers, Immortals, Brood Lords, Mutalisks, Infesters
Corruptor – Ground to Air, Large Air armies (non-massive), AE Spells, Mutalisks, Vikings, Hydralisks, Stalkers,
Mutalisk – strong AA units, turrets/cannons, AE spells, Hydralisks, Stalkers, Thor’s, Templar
Siege Tank – Open Ground, Fast Melee Units, Lack of Unit Support, Immortals, Banshees, BattleCruisers, Pheonix, Void Rays, Mutalisks, Brood Lords
Hellion – Armored Units, Air-to-Ground, Air to Ground, Tanks, Marauders, Stalkers, Immortals, Colossus, Roaches, Ultralisks
Ghost – Detectors, Large Armies, Thor, Marauders, Helions, Stalkers, Colossus, Roaches, Hydralisks, Ultralisks
Reaper – Large Armies, Fast Units, Ranged Units, Any non tier 1 melee unit
Marauder – Air-to-Ground units, anti-armored units, Choke Points, Stacked Armies, Tanks, BattleCruisers, Colossus, Carriers, Templar, Ultralisks, Hydralisk
Marine – Open Areas, Late Tier Units, Splash Damage, Tanks, Marauders, BattleCruisers, Zealots, Sentries, Colossus, High Templar Carriers, Roaches, Ultralisks, Infesters
Banshee – Air to Air, Large AA units/groups, Magic Casters, Air to Air Units, Thors, Hydralisk
Raven – If Left Alone, Long range AA, Air to Air Units
Medivac – If left alone, Fast or Long Range AA, Any support unit
Viking – Ground-to-Air, Weak in Ground to Ground Combat, Thors, Marines, Stalkers, High Templar, Hydralisks, Mutalisks
Thor – Fast Melee Units, Lack of Unit Support, Poor Terrain, Movement Speed, Battlecruisers, Tanks, Void Rays, High Templar, Infesters, Corruptors
Battlecruiser – Long Range AA, Magic Casters, Viking (out ranged), Void Rays, High Templars, Corruptors