Imagine the year is 1998, and to date the best FPS games you've played are Doom, Duke Nukem 3D and Quake. Suddenly out of nowhere comes a new FPS game from Looking Glass Studio where you...try not to kill anything? Wait, what?
Set in a sort of medieval steampunk world, Thief is the story of Garrett who is a Thief (Surprise!) and gets caught between 2 opposing factions (Pagans and Hammerites) in his quest to steal as much loot as possible. The story is actually much more complicated than that, told through in-game notes, monologues and hand drawn cutscenes. But that is the basic gist of it. The story is great and Garrett is one of the best protagonists in a FPS game or otherwise. But the gameplay is really what blew me away at the time.
Though probably not the first stealth game ever, Thief certainly was (and still is) one of the best. It was the first to use light and sound for enemies to detect the player. You could use water arrows to put out torches, moss arrows to mask your footsteps and even a blackjack to knock your enemies unconscious. Enemies would react to blood stains, loud noises and shouts from their friends. Yeah that's pretty common stuff now, but in '98 it was fucking unheard of.
The level design was also pretty open, with multiple paths and lots of extra loot to steal. Depending on the difficulty, you had extra objectives to complete and your path through the level might change. Between levels you could buy more tools and items to help you on your next mission. There are just so many elements here that make a great game, and Thief combines them all to form a superb complete package. You can find it's influence in Splinter Cell, and pretty much every stealth based game that came after.
Though the graphics hold up about as well as anything from that era (very poorly) and the stealth elements it pioneered have been reused ad nauseum, Thief is still well worth playing. It's also fitting that one of the best games of all time should spawn one of the best sequels of all time.