Most of the above, but most especially jet exhaust.
And the cold, sharp smell of pure oxygen in a freshly cleaned oxygen mask. I don't get to smell that one these days, but it's right there in my memory almost 50 years later.
The memory is so strong that I still know we cleaned our masks with benzalkonium chloride, and its faint odor combined with the rubber of the mask and the supposedly odorless oxygen made a very heady combination unavailable anywhere else but a cockpit.
