Thief 2X: Shadows of the Metal Age may not be official but it is an expansion worth playing through.
Despite some hiccups here and there, Sly Cooper: Thieves in Time is a fun stealth platformer and one of the best games in the series.
Sly 3: Honor Among Thieves is a fun, albeit bloated game that doesn’t quite reach the same heights as its predecessor.
Sly Cooper and the Thievius Raccoonus is a fun but easy platformer that shows a lot of potential.
Thief: Deadly Shadows is a solid entry in the franchise with some notable improvements to the formula.
Thief II: The Metal Age gives you more of the same great gameplay and with all the annoying stuff stripped out. It’s a far better experience.
Thief: The Dark Project is one of those “must play” games. Even with its few drawbacks, it’s one of the best games in the stealth genre.
Requiem is a well made story-driven mission that contains everything great about Thief gameplay.
The Dark Mod is a Thief game without actually being a Thief game.