The Mad Titan is seemingly unkillable, coming back to menace Earth again and again, but while Iron Man and the Avengers will always stand against him, it turns out Thanos doesn't actually need to worry about the two god-tier suits fans might have assumed Tony Stark could use against him.
It is quite hard to declare which versions of Iron Man's suits are the most powerful. Tony is, after all, a man obsessed with building weapons and constantly improving them. This obsession constitutes one of the character's most interesting and controversial traits and has been explored many times over the years. Stan "The Man" Lee himself once said that he created Iron Man to be completely unlikable; a narcissistic weapons merchant who made his debut at a time when young readers hated war and militarization. And yet while fans may debate the relative merits of the Hulkbuster vs the Thorbuster, two sets of armor stand out as the apex of power: the Godkiller and the Godbuster. And yet, if Thanos makes another attempt at conquering the universe (or at wiping half of it out), he's free to totally ignore them.
The Godkiller armor was not actually built by Tony. It was the weapon of the godlike race of the Aspirants, used in their war against the rebellious Celestials in the First Cosmos. After learning of its existence, the Rigellian Recorder 451 decided to use it as a deterrent to protect Earth, genetically engineering a human child to be the pilot that could control that enormous power. Believing that child to be Tony Stark (it was actually his secret brother Arno), 451 connected Tony to the suit, only to find out he was unable to pilot it. Driven to despair from the failure of his plan, 451 transported the Godkiller to another dimension. Despite being a weapon so powerful that it could kill Celestials, the Godkiller is now presumably lost forever. Tony's attempt at recreating it proved to be unsuccessful, and he had no way to pilot the original, even under threat of death.
The Godbuster armor, on the other hand, was actually built by Tony and can easily be considered his most powerful creation. While trapped inside eScape, a VR world created by himself, Iron Man had to face the world's AI gone rogue, Motherboard. In that virtual environment Motherboard had godlike powers, but so did Tony, who used the unbounded creativity and infinite resources he was granted in that world to build a weapon that could kill a god. After escaping into the real world, Tony immediately built the Godbuster armor there to defeat the Controller. However, he was able to do that only because he still remembered the schematics from his time inside eScape, but they were quickly fading from his mind. He described it as "like trying to hold onto a dream once you wake up." Tony then blew up the Godbuster, scared by its excessive power, and the design disappeared from his memory.
Iron Man's most powerful suits are lost forever, and they could both be missed in the near future. While Tony is always capable of building the next, great suit of armor, the Godbuster and Godkiller are off the table, each relying on schematics he doesn't have and can't recreate (not to mention immense resources.) Ultimately, this is what makes Tony a hero - his genius allows him to access untold power, and yet he (usually) has the wisdom not to hold onto it.
While fans are right that these suits represent the pinnacle of Tony's armory, they were one-time weapons that writers made sure he wouldn't have recourse to again. The next time Thanos strikes, Iron Man won't have the option of turning the Godkiller or Godbuster armors again him - though fans can rest assured he'll express his genius with something new (albeit probably not quite as powerful.)
