Well, there is actually a lot of games that make me fell this, but for the sake of the thread I'll go just with THE one.
The Legend of Dragoon (oh, what a surprise BraLoD).
It's just that everytime I play it... it's exact this, pure magic, I get into it, immersed, and I get myself actually smiling a lot while playing, and doesn't need to be funny moments, it's a pure expression of joy, passion. I know everything that will happen, actually almost every dialogue line as soon as they start, and even so, it's an absurdly rewarding experience. And I like to do that completely alone by the way, any distraction is unacceptable! XD