Late at night, when in the middle of a gig, with a good band behind me, and we all hit a fantastic groove and just go with it, and the audience are right there too.
For solo times, late at night, when I've managed to replenish my levels of interest in things (when I don't get to watch TV, play games, play the guitar and read multiple fiction books, I get crabby) and then i can settle in to writing my own books, which are always better in this state.