Thats the nice thing about the first Starcraft their wasn't a unit that was overpowered. You could find something against anything...at least that is my experiance and even lower level/tech units where allways still usefull.
I've only ever played Starcraft online once, and I won by building ONLY zerglings (and because I got lucky, okey):
Battle was me (zergs) versus, iirc, a terran and a protoss player. Two goddamn good players too, it seems. It went that way:
Phase 1) At the begining of the battle, I just got all these zerglings, moved them in the middle of the map (the map was like a big cross with each player at one corner), and buried them all there. Then I repeated and repeated that again ( barely three of four times actually, but I still got a lot of them there ).
Phase 2) Ridiculously early (from my point of view, at least, I guess I just suck at base management), the two other players moved toward the center of the map, with, dunno, crazy stuff, carriers and the **** (I was like "how the heck did they get that already?"). Anyway, I assume there was to be some "perfect way" to get a moving army ready soon, coz they both got theirs at the same time.
Phase 3) They meet at the middle (just above my hundred or so poor zerglings ****ting their pants, hoping there's no stealth detector or anything around) and they just battle it out there. I guess I got lucky there, no flying units remained from that little skirmish (and probably not many firebats
Phase 4) when they were about done, I just unburied my zerglings (totally crushing the few survivors still fighting in the process), launched all of them at one base, and all the others I had gathered at my hive during that epic "battle of the center" at the other base.
Phase 5) I probably didn't have enough zerglings to crush them completly in one go (don't remember), but I hit them hard enough so they couldn't recover (I do remember just having a never stopping stream of zerglings moving to both bases at some point, and me just producing more and more of them as they died at the other end w/o even checking the enemy bases, I probably just put the spawn points there - was there one in starcraft?). I remember looooooooooooooooots of zerglings lost their poor lives attacking these bases (they had goddamn strong defenses :/)
Conclusion 1: I won because I sucked at SC and the two others were too good
Conclusion 2: I decided it wouldn't work again and I would not bother learning the fine art of " how to build up a strong base and strong army in less than 15 minutes" and just never went on battle.net again.