While normally out of respect I would just mention the whole series, I will choose only Baldur's Gate II: Shadows of Amn this time to be more true my views. Best game I have ever played without question.
I cared about my party of NPCs. I cared about the romance I had with on of them. I cared about the story. I cared about defeating that magnificent bastard of a villain, Jon Irenicus. I cared about the voice acting done by David Warner for said villain. I cared about Grey DeLise voice acting for a certain romanceable Drow NPC. I felt like my character was truly powerful and important, and really driving the story along in an important way, not being driven by it. I adore the combat, the dialogue, the layout and the music. I cared about the plot. That game is perfect for me.
While I always respect the truth that there is no No1 game for everyone, there is no game better than Baldur's Gate II. Its one of those very special games that just deserves to stay in that golden area for games that exemplify the pinnacle of what their genre is capable of, never to be "beaten".