A favorite of the author, and is probably the most iconic non-main-character-centered case. Assistant inspector Sato's tragic past and Takagi's memorable elevator scene with Conan definitely puts this on the top of the iconic list. So iconic that the fans AND coworker of the author wants Matsuda (who died in Sato's past) to comeback.
Not only the longest main-plot related case involving a battle of wits against the black organization, but also among the longest cases in DC history period.
Scarlet would actually be great. The back and forth switches between the scenes on the road and at the Kudo house were amazing, You-Know-Who turning out to be alive was great. Iconic for sure.