The very first collection, opening with Calvin talking about catching a tiger, ending with Hobbes observing -- upside, munching on the bait sandwich -- that they're foolish like that.
He had the characters and the art down from the very first. Most of them get introduced here, not only Calvin's family, but Suzie, his teacher, Moe the classroom bully, Rosalyn the baby-sitter who can cope. . . and, of course, Spaceman Spiff. Some of these will fade out -- I don't know if his brief membership in Boy Scouts ever returns -- but Calvin's gloriously hyperbolic imagination is what we all remember.
It has the sequence in which Hobbes gets lost, too, and firmly puts a question mark on whether Hobbes is real or imaginary with Suzie's cookies.