The X-Men's Cyclops once took on Iron Man and Thor - at the same time. It's become something of a truism that, whenever different superhero teams meet, they inevitably come to blows at first before ultimately teaming up. The relationship between the X-Men and the Avengers has only been a particularly fraught one, though - and for good reason.
Where the Avengers are celebrated by society, the X-Men find themselves sworn to protect a world that hates and fears them. They've resented this, and at times becomes angry that the Avengers have stood by and watched while the entire mutant race was threatened; the X-Men will certainly never forgive the Avengers for failing to intervene and prevent an act of genocide on Genosha. Meanwhile, the Avengers for their part have watched the X-Men's various alliances with distrust, understandably wary of so-called heroes who would welcome Magneto as a teacher of the next generation.
The hostility between the X-Men and the Avengers runs all the way back to their first meeting in X-Men #9, an issue that saw Charles Xavier summon his students after he had discovered an alien threat named Lucifer. The Avengers, too, had discovered the presence of a terrifying extraterrestrial power, and they rushed to confront it. Too late Xavier realized Lucifer had created a booby-trap, meaning an entire continent would have been obliterated if his heartbeat was disrupted. The Professor was forced to command the X-Men to stop the Avengers - and Cyclops followed orders in a very original fashion.
Cyclops had correctly identified the two most powerful Avengers, and his opening optic blast disarmed Thor and left Iron Man stunned. It was an admirable tactic, made even more effective because back then Thor would transform back into Donald Blake if he was left without Mjolnir for long enough; Cyclops didn't know it, but by blasting Mjolnir out of Thor's hand he had come close to taking the Thunder God down with one shot.
This feat is all the more impressive when you consider Cyclops was essentially a rookie, and he had only just stepped forward to lead the X-Men for the first time. Consequently, this should be considered one of the first examples of Cyclops' tactical skill, the instinctive ability that would allow him to lead the X-Men to some of their greatest triumphs. In the end, of course, the X-Men and the Avengers called a truce; but Scott Summers had made sure the mutants got some good hits in first.