Defining the "most popular" Wii game depends on the metric: units sold, critical acclaim, or cultural footprint. Several titles stand out as defining the system.
Wii Sports and Wii Fit transcended gaming, appearing in nursing homes, rehab centers, and family living rooms worldwide. Mario Kart Wii defined couch/online multiplayer for a generation. Super Mario Galaxy is frequently cited in "greatest game ever" conversations.
Verdict: Wii Sports wins on raw reach; Mario Kart Wii on multiplayer longevity; Super Mario Galaxy on artistic achievement.