Mid-way through WoL and again shortly after launch of HoTS.
I played BW a lot as a kid, and the fundamental concepts carry over pretty decently. Always build workers, don't get supply blocked, expand as fast as your opponent lets you get away with, never let your resources bank, upgrades are always worth it, etc. etc.
As I recall someone in WoL actually went from bronze to masters building literally no units except marines, medivacs, and scvs to prove the point raw mechanics are all you need to carry you up the ladder.
To be fair marines and medivacs are amazing units in WoL. The same likely wouldn't apply with most other two unit combo. Roach ling maybe, maybe blink stalkers.
I played BW a lot as a kid, and the fundamental concepts carry over pretty decently. Always build workers, don't get supply blocked, expand as fast as your opponent lets you get away with, never let your resources bank, upgrades are always worth it, etc. etc.
As I recall someone in WoL actually went from bronze to masters building literally no units except marines, medivacs, and scvs to prove the point raw mechanics are all you need to carry you up the ladder.