Living
I think that the most exhilirating feeling in the world is that feeling that is akin to flying, when you're glued to your seat, anchored to the earth, and whatever it is you're experiencing lifts you up beyond the limits of imagination and physicality. It feels like your heart is about to explode, but that would just propel you further, so you wish it would. You're caught in a headlock, and all you can do is sit as tears run down your face and you have no clue how to express what you're feeling. I still haven't decided if it's better to have someone next to you or not; I suppose that would completely alter the experience, but then you would have something to clutch at as you bite your lip and try not scream with the momentum.
And then it's over, and all you can do is push yourself forth through the exhaustion and use the resulting feeling to feel as though the world is a kind place, more beautiful than anything the imagination could ever in a thousand years provide.