The games industry is extremely competitive--sometimes it's not enough for a game to be good in order for it to be a commercial success. In spite of their great qualities--and in spite of critical acclaim--each year, certain games fail to draw the kind of public attention they seem to deserve. The cause of this can be bad marketing, bad timing, an overly ambitious design, or something else. Here is the Best Game No One Played in 2001:
Clive Barker's Undying

Publisher: EA Games
Developer: EALA
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Whether due to its lack of a multiplayer mode or for some other reason, EA's spooky and atmospheric shooter utterly failed to meet the publisher's sales expectations. Undying had all the makings of a hit--this occult-themed action game is immediately accessible to shooter fans, yet it has its own unique look and feel. Undying compares favorably with the genre's classics, yet it didn't put a dent in the sales charts, causing EA to cancel the free multiplayer add-on planned for the game, cancel a translation of Undying for the PlayStation 2, disband the design team, and therefore eliminate any possibility of a sequel. It's too bad that great games like Undying can't sell commensurately with their high quality. But for those of us who did play it, we're better off for it.
Other Nominees:

Anachronox
Independence War 2: Edge of Chaos
Jumpgate
Shattered Galaxy