There are so many possibilities...
I think seeing someone completely in the moment and just in their element and being 100% themselves is hard to beat.
I watched a friend give a piano recital the other night and he was just lost in the music, I kinda saw him wake back up to this reality when he was done playing haha. I think people are like electrons, they act a certain way when they are observed and a certain way when not. But observing someone just "doing their thing," not giving a fuck and being totally in the moment even when you're watching is so incredibly beautiful.
And think about seeing ppl rolling just dancing their ASS off at a rave, haha I love when you see someone with their eyes closed just wilin out