Tony Stark’s strongest suit in the MCU is generally considered the Mark LXXXV (Mark 85) , his final Endgame armor. It’s the most durable version he built, and it could handle Thanos-level combat and the power of the Infinity Stones far better than his earlier suits.

Material used

Tony did not usually build his suits out of a single “strongest element.” His earlier armors used iron, then titanium or titanium-gold alloys, while the Mark 85 used advanced nanotech rather than being a pure vibranium suit.

About vibranium

Vibranium is often described as the strongest metal on Earth in Marvel, but Tony never fully made a standard Iron Man suit from it in the MCU. So if you mean the strongest element associated with his suits, people usually bring up vibranium , but if you mean the strongest actual Tony-built suit , it’s the Mark 85.

Quick answer

  • Strongest Tony-built suit: Mark LXXXV / Mark 85.
  • Strongest material people associate with what he could have used: vibranium.

If you want, I can also give the answer in a one-line fandom-style version.