Here are five other games that could make legitimate arguments for a spot on any similar top-five list, in chronological order of release:
Delusions - A compelling work of science fiction that really seeks to mess with your head. Disturbing and innovatively structured, and undoubtedly will remain on the mind of the player well after it is completed.
Spider and Web - The game that beat both Anchorhead and Photopia in the 1998 XYZZY's, would be #1 on many lists. I personally was let down by the final ten minutes of the game, but the storytelling device up to that point is second to none in its brilliance, and the story itself is extremely intriguing.
Varicella - Adam Cadre's espionage/horror/humor game, referred to by Duncan Stevens as "possibly the most disturbing work of IF ever written." It takes place in a small Italian palace, and features all manner of murderous intrigue. The game ends after a set number of moves, and that number is small enough to demand numerous replays in order to fully understand what is going on in this palace. The size of the game file is almost as large as Anchorhead.
Worlds Apart - Suzanne Britton's sorely underrated fantasy adventure requires a lot of patience, but is ultimately absorbing and extremely unique. It also introduces a previously unforseen level of depth and backstory for NPCs.
Galatea - As much of a "work of art" as interactive fiction can be, this game consists of nothing more than a conversation with an inanimate statue. Sounds boring, until you play through it more than once and begin to realize what an amazing job of coding Emily Short has done. The titular NPC has more complexity and psychology than any NPC in the history of IF. A remarkably unique experience.
Tune in tomorrow for a profile of five important names in the IF community!