Heavy assault units are extremely powerful units used by Commanders to attack bases head-on. They are generally slow-moving, expensive to produce, and have to be supported by supplementary and specialist units to protect them against certain forms of attack that they cannot defend against.
Perhaps the only mass-used heavy assault unit of the Great World War II was the original Soviet Mammoth Tank. The Soviets also researched ultra-powerful Super tanks to crush the Allied forces, but the tanks' creator defected and the prototypes were destroyed by preemptive Allied strikes.
First Tiberium War
The GDI used the X-66 Mammoth tank, a reverse-engineered variant of the Soviet mainstay, though the tank entered into combat late, as Nod attempted to destoy the prototypes before they could be mass-produced. Nod preferred numbers to brute force; producing large amounts of M2 Bradleys rather than comparatively small amounts of heavier tanks.
Second Tiberium War
The GDI's most powerful unit during the Second Tiberium War was the Mammoth Mk. II, though it's production costs were enough to prevent their mass mobilization; most commanders were only granted access to one Mammoth walker at a time, and had to use lighter Titan walkers as the backbone of their armored divisions. Nod's Cyborg Commando was likewise capable of destroying a base by itself; yet they were significantly rarer than weaker Nod units. CABAL's Core Defender was the most powerful unit in the world upon its creation towards the end of the Firestorm Crisis; however, it's lack of effective protection against enemy aircraft proved a fatal design flaw.
Third Tiberium War
In the modern age, GDI fields large numbers of Predator medium tanks, supported by enormous Mammoth 27 tanks, and recent intelligence indicates the usage of a massive "MARV" mobile fortress, though yet again, it's expense meant it was extremely uncommon. The Brotherhood of Nod fields fleets of light Scorpion tanks to handle assaults early in any battle, but can also produce highly destructive Avatar walkers once a sufficient technological and resource base has been acquired. A powerful war machine known as a Redeemer also appeared in Nod's arsenal during this time. The Scrin make use of combined forces of Devourer tanks, Seekers, and Annihilator Tripods, while also fielding airborne assault units, such as their Devastator Warships and Planetary Assault Carriers. Another unique assault unit was the Eradicator walker. They also are willing to send their Motherships into battle; these massive space ships are capable of extreme destruction.
Fourth Tiberium War
The Mastodon and the new Mammoth Tanks will serve as the Heavy assault units.