all units are good ,for example lings had little HP but it attacks very fast,upgrade its speeds,1v1 zealot, ling can almost take it out. all units had their own specialties, like ghost/vulture good against zealot. weaker units always had special strengths or abilities so they can be balanced to other units. all the abilities all useful and powerful too. u just have to use them in the right time.
In my own opinions the most powerful units are carriers(4newb), BC(4new), Guardians.
And i hate carriers,their little fighters are annoying.