This is such an awkward question lol. Probes are by far the best harass (shields, pylon blocking, cannon rushing), drones have the most utility (Stealing gas, Regen HP, can make proxy hatches etc) and scvs are the most useful for a fight (with repair, most hp, etc). Basically I'd say probes are the most broken in every game. Drones are pretty good for worker vs worker wars, although that's because probes are terrible straight up and the way zerg gets more drones quicker than terran early on (talking drone all ins here xD), I'd say drones are generally second best at everything. Not quite as good as scvs for battle and not quite as good as probes for harass.
Therefore, if you're a cheesy all-inner scvs are the most OP worker, if you're a macro player trying to gain an advatage probes are the most OP and finally if you're a player who either A) Wants a mix of both or B) Love not having to shift click you're workers back to minerals when you build something, then drones are the most OP. I think I'm gonna vote for drone though just because Drone rushes are actually the most baller thing.
In Brood War, it was the probe...NonY vs IdrA...memories
In SC2, I don't understand how you could argue against the unit.
I remember from IPL3...the question was "what would you like to see in HotS"
SeleCT: "a melee unit for terran"
oGsMC: "melee unit? you mean SCV?"
seriously, it's the only worker that's legitimately awesome to fight with...try holding a thor rush with SCVs pulled after you nexus first, try killing 2 banshees with 6 stalkers, try...but they stab their fusion cutters in and your hopes and dreams are decimated in the process.
I can't begin to imagine what zergs go through with bunkers
everyone knows the age-old tale of that one probe you didn't find in your base...and protosses know that feeling when your probe gets sniped in PvP. but at least those suckers can't fight...except in HuK vs HasuObs on Terminus from NASL, that was an epic game
SCV are the most OP
pull them with a push to make it significantly more effective
more hp
can be healed AND repaired... like are they bio or are they mech ~.~?
And they can repair during combat
Not to mention how they really love to hide in spots where your drone can't actually reach them when they are building a bunker at your natural TTTTTTT
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