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.
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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.
Legally, there aren't any options, but torrent sites are your next best choice. For example, you can google "Starcraft torrent download" or something similar.
Starcraft was co-developed by Saffire and Blizzard Entertainment.
Zerg is an alien race in Starcraft.
If Blizzard did make world of Starcraft there would hardly be a difference between World Of Warcraft and World Of Starcraft since Starcraft and Warcraft are very similar games. so there is really no point of making it.