It’s not the first appearance of Big Bad Thanos, but it’s one of his first ones, and it’s in the first story arc that he appeared in…And he’s hiding behind a chimney. (This story takes place during Captain Marvel #31-32.)
He got cooler later. Actually, he was cooler in this very same story. Hiding behind a chimney is totally undignified.
Also: Swordsman notices Vision is starting to dig Mantis, and he calls him out on it right in front of Scarlet Witch. Viz should have decked him for that.
Creators: Steve Englehart and John Buscema