THIEF
Probably my most favorite game. Of course I'm refering to Thief Gold, Thief Metal Age and Thief Deadly Shadows. Thief 4 was a failure and I didn't have as much fun as I had with the other titles. They called Garret to a complete different character and barely referenced the Keepers. And I'm not very Fond of the Watch.
I played Thief The Dark Project before Thief Gold. It was a point of no return. The mystery, the occult like elements, the secrets of the pagans, the religious fanaticism of the Hammerites, the silence, the shadows, it was a formula that kept me hooked for hours.
Then came Thief 2 The Metal Age with Karras, his machines and his mad plans. It was again many hours of joy and pleasure.
Because my machine at the time had low specs, I only played Thief 3 Deadly Shadows much later. With new mechanics and camera view it felt different, but not less fun as the other titles.
Once in a while I return to the franchise and play the triology. Though, TDP/Gold still remain my favorite of the series.