First, Ill start with the race that I play the most, the Orcish Horde. The Horde possesses the game's most powerful ground units, including the savage Grunt and gargantuan Tauren. Ironically the orcish army does not do very well in stand up ground battles and relies more on hit and runs and hero kills to win games. In fact one of its supposed weaknesses, air, tends to be one of the more used units. So where does this leave this race with 'superior' ground units. Well at least its champion ground unit, the gargantuan Tauren is left sitting on the bench because the Tauren, in my opinion, is currently one of the hordes worst units. He is the only orc unit that never really gets used in solo games.
Next we will look at the Undead Scourge. Argueably UD doesn't really have any units that are horrible, but run more along the lines of lacking synergy with the mainstay heros of UD, DK/Lich. One of their racial strengths listed is the ability to use corpses, which UDs do use with their rods of necromancy, however, when is the last time you saw a good player use consume corpses with ghouls, let alone Aboms? One would think this would be a corner stone of the race as it is one of the key things that make the Undead Scourge unique. So the UDs most useless unit? Banshees/Necromancers. Not that these units are completely useless however, some of their racial idenity in terms of synergistic abilities seems to be lacking.
Next we will examine the units of the Night Elves. The Night Elves of Kalimdor are a mighty race that emphasizes mobility, ranged firepower, and spellcraft. They do not have the brute strength of other races, but their skills with bow and magic more than compensate for this deficiency. Looking at their racial characteristics the Elves seem to closely fit their description. All of their ranged units are good, especially considering cost and build times. Also, keeping close to the racial description, they lack brute strength(bears are argueably pretty decent melee units) and thus leading to the Night Elves most useless unit, the Mountain Giant. When is the last time you have seen of these things used effectively in a solo game?
Lastly we will examine the Human Alliance. The Human Alliance is the most versatile army in Warcraft III, with good ground and air troops, excellent siege capability, and powerful spellcasters. In terms with racial identy the Humans stick very closely to their intended racial identity. Because of the fact that they are versatile it is very hard to find a bad unit in the Human arsenal. For instance Gyros are good massed against air. Seige tanks are decent air and can be used to raid bases and to be a general nusance. So what is the Humans worst unit. I would have to go with the footman. Although humans cant live with out these weak units, they are by far humans worst unit as they really have no use besides the fact humans need them for early denfense and creeping purposes and are quickly rid of once the humans has teched to its much more valueable units.
I will not be poisoned by your bitterness.