I was awestruck going through my first playthrough of Half-Life 2, when it came out many years ago. So cinematic and engaging! Then, there was Bioshock! Amazing atmoshere! Then, Far Cry 1,2...Amazing graphics, getting closer and closer to reality with its tropical island, and African environments.
My #1 favorite? That's tough...I might say the Battlefield series...since I've played it by far the most, on a frequent basis...all the sequels, all these years.