Marx was central to South Africa's 29-24 victory over New Zealand in Johannesburg, giving the world champions a 2-1 lead in the four-Test series.
The 92-cap hooker produced an outstanding all-round display, throwing accurately at the lineout, winning a crucial breakdown penalty and scoring from a driving maul late in the contest.
Speaking on the I Love Rugby podcast, Smit highlighted the range of qualities that make Marx such a difficult opponent.
Smit, who was a hooker and led the Boks to 2007 World Cup glory as captain would himself be in the debate as to the country's best ever as well as the likes of Bismarck du Plessis, James Dalton and Schalk Brits.
"There's a couple of things about Malcolm that make him the best in the world, certainly the best who's worn the No 2 shirt in South Africa," Smit said.
"He's a beast. He's a monstrous man. He's big, athletic, fast. He carries well, he steals, he scrums, he's got an unbelievable throw. There's no real chinks in the armour."
The former Springbok captain also highlighted Marx's impact on opponents, saying he must be "the bane of existence for every opposition team that plays against him".
Yet Smit noted that Marx remains widely respected away from the pitch.
"But I don't think there's one person in world rugby who doesn't like him as a human afterwards, that can't wait to have a beer with him. And I think that says a lot about the guy," said the former Bok captain.
Smit also reflected on the importance of South Africa's scrum after New Zealand took a 12-5 lead.
The Springboks responded with a first-half penalty try from a dominant scrum, which Smit felt was crucial to restoring confidence.
"Every South African, every 'green goblin', was waiting for the scrum to deliver," he said.
"South Africa need that weaponised scrum to create their confidence and get that psychological benefit."
With South Africa leading 2-1, Marx will have another chance to underline his importance when the teams meet in the fourth and final Test in Baltimore on Saturday.