it depends on what you are against, but the Thor and thew Battle Cruiser are both pretty strong
The answer to that is debatable as every unit has it's strengths and weaknesses and every unit can be countered with another, this makes the game balanced.
the strongest units is who you will se
For Terran, the ghost is the cloaked unit. For Protoss, the observer and dark templar are cloaked. Zerg has burrowing instead of cloaking, and all zerg units can burrow if the upgrade is purchased.
It is highly suggested that you go and check The Shokz Guide (link below). It is the complete Starcraft 2 Mastery Guide. The Starcraft 2 Guide is an in-depth guide teaching you how to master the Terran, Protoss, and Zerg. Great Guide! You can become the commander of an army, and control your units (for example: where to go, which enemy unit they should attack, etc.).
1 - Probe 2 - SCV 3 - Drone protoss- fully upgraded Dragoons or carriers with arbiters backing them up(note, Dragoons get EASILY overwhelmed by zerglings, so bring some zealots or reavers.) Zerg-Guardians with scourges or devourerers, hands down. Terran- Goliaths.
There is no single most useful Zerg unit; any unit can be useful, useless, or somewhere in between. Where a unit falls is dependent on the current situation. Compare to the game Rock-paper-scissors, where rock is useful against scissors, but useless against paper. Of course the unit relationships are more complicated in the StarCraft games.
unlock it
Cyber-Inquisitor
In the StarCraft Universe, Kakarus resemble a pterosaur dinosaur that are native to the Dark Templar's planet Shakuras. Their broad wings allow them to travel swiftly across the skies of the desolate planet. The player can interact with the neutral unit by planting parasites on these "spies" with Queens from the zerg species. They can also be killed by any unit that can attack air. The unit has 60 Health Points and is considered a small unit in the StarCraft platform. As a neutral unit, they do not attack players and players cannot see them unit they cross paths. Their appearance in StarCraft 2 is still unknown but Blizzard has supported wild life preservation, ergo the extinction of the Kakaru species is rather dubious.
They deleted the tempest and replaced it with the carrirer because of emotional ties with the unit
Click on a unit, hold control, then press a number. When you press that number again it will select the unit, group of units, or building you previously selected.
Press A on the keyboard or press the attack button, then left click on the unit or building you want them to attack.