His own title was usually written to make things challenging even with lesser opposition, but I recall Strange defeating Nightmare pretty handily in Dr. Strange #13, ditto by merely opening the Eye of Agamotto at the beginning of Dr. Strange #50, and easily making short work of Mordo's lieutenants in Dr. Strange #56 by basically letting his house eat them.

Also sticking out in my memory are (1) the old issue of Defenders where as the Red Rajah Strange took out Solarr, the Rhino, and the male half of his own team as if they were toddlers, (2) an issue of Namor where the titular character, Iron Fist, and possibly Misty Night and Colleen Wing were fleeing from some über-powerful sentient plant monsters and Strange disintegrated them effortlessly, and (3) Fantastic Four #243, in which Strange waved his hands and knocked Galactus into a coma with a single spell.