Thief one and two were absolute gems. Excellent story driven missions which all comes together with an added twist. The gameplay was also good and the level design (for its time) created some very immersive experiences.
Sadly the development team broke away, as most of the best ones do :(
Thief three, although a great game, lacked in the gameplay, and when released had an AI difficulty reset problem. However, it can be fixed by running a program called "Andrew's Thiefbot" read his addendum in regards to the latest patch released by the developers of Thief 3 not fixing the problem.
Apart from the above problem (which can be fixed), the third title maintains the quality of stories and twists, as well as a great atmosphere.
It would make me happy to see Thief 4 in the pre-order charts :)