Thief was pretty revolutionary for stealth games - putting the player in a FPS position and eliminating many of the artificial display systems (radar, enemy state indicators) in favour of having to 'read' the lay of the land and determine enemy state by direct observation. Also, one of my favourite games ever. As one of the Splinter Cell team, this was the game that influenced our gameplay design the most, along with Deus Ex, a sort of spiritual successor in many ways. Open layouts, excellent AI and VO, amazing music - this should be on any game designer's must-play list even after all these years.
