My answer to this question has changed over the years. (I can remember when my most honest answer would probably have been "during SEX"!)

Later, during the years I spent a lot of time camping in the mountains, I felt most alive- alert, senses open, my heart-in-my throat alive - in the middle of the night standing alone in the quiet watching the world around me and there was not one sign of human habitation.

Nowadays, with the decrepitudes of age and its concomitant inward pleasures, I'd have to say that it's in the middle of music. I can lose my self and still feel totally alert and receptive.

Where do YOU feel most alive?