Who Is Invincible's Strongest Viltrumite? It's Complicated
Several characters in "Invincible" could rank among the strongest comic book characters ever — all of whom are part of the alien conqueror race, the Viltrumites. Omni-Man begins the series as the most powerful being on Earth, killing the Guardians of the Globe easily before turning on his son, Mark Grayson. Meanwhile, Invincible himself is no slouch in the strength department — throughout "Invincible," he progressively gets stronger as he comes into his extraordinary powers. However, one person sits atop the list of the strongest Viltrumites during most of the comic: Thragg.
Grand Regent Thragg first appears in "Invincible Returns" #1 (by Robert Kirkman, Ryan Ottley, FCO Plascensia, and Rus Wooton) as the ruler of the Viltrumite Empire. Thragg fights for the survival of the Viltrumites via deadly means, putting him at odds with Invincible and several other heroes on Earth. During his earliest appearances, Invincible tries to fight Thragg, but even his powerful blows don't make a small dent in him. He seems invulnerable, even to the most potent attackers. Later, when the Viltrumites learn Thragg doesn't belong on the throne, he nearly takes down Omni-Man in a battle before being exiled by his own kind. Upon his return to try to stop the Viltrumites, Thragg kills the powerhouse Battle Beast, Oliver Grayson, and later kills Omni-Man. In his last stand, Thragg nearly kills Invincible, but in an intense final battle, Mark kills him (while nearly dying himself), with his body getting engulfed by the sun.
Thragg is the strongest Viltrumite in "Invincible." However, the comic leaves the door open for a different character to take his place.
Mark may be the strongest after the series finale
Despite Thragg's strength being unrivaled throughout most of "Invincible," when the series reaches its end, there's an argument to be made that Mark Grayson has taken the title of strongest Viltrumite for himself. As the story ends, "Invincible" flashes forward 500 years into the future, where the titular hero becomes Emperor Mark Grayson of the Viltrumite Empire. He's brought peace to his people and rules alongside his wife, Atom Eve, with their daughter, Terra.
While his powers aren't fully revealed, one of the most significant indicators of strength among the Viltrumites is the passing of time. Considering Mark is hundreds of years older and stronger, defeated Thragg in their past battles, and sits atop the Viltrumite throne, there's a case to be made that he ends up the most powerful Viltrumite as the series concludes.
Mark might have leapfrogged Thragg in strength as the "Invincible" story ends, but it's somewhat open to interpretation. It will be fascinating to see what approach the "Invincible" animated series on Amazon Prime Video takes, as whenever the show finally concludes, it can answer the question of who is the strongest Viltrumite more definitively than the comic book ever does.