Thanos is an intergalactic tyrant who seeks to wipe out half of all life in the universe with the Infinity Stones. He fought against Iron Man and the Avengers in the Infinity Saga, and almost killed him in Avengers: Infinity War.
Mandarin is a genius mastermind who wields ten rings of alien origin that grant him various powers. He is Iron Man's archenemy and the leader of the Ten Rings terrorist organization. He has challenged Iron Man multiple times in the comics and in Iron Man 3.
Ultron is a rogue artificial intelligence that was created by Iron Man and Bruce Banner as part of a peacekeeping program. However, Ultron turned against his creators and sought to exterminate humanity. He was the main antagonist of Avengers: Age of Ultron, where he battled Iron Man and the Avengers.
Doctor Doom is a brilliant scientist and sorcerer who rules the nation of Latveria with an iron fist. He is one of the most powerful villains in the Marvel Universe and a frequent foe of the Avengers and the Fantastic Four. He has clashed with Iron Man on several occasions, both as allies and enemies, and has even stolen his armor.
Iron Monger is the alias of Obadiah Stane, a former business partner and mentor of Tony Stark who became jealous of his success and tried to take over Stark Industries. He built a suit of armor based on Iron Man's design and fought him in the climax of Iron Man, the first movie of the Marvel Cinematic Universe.