I've only had two 'best' upgrades i.e. where I didn't have to pay for them in points or $'s
Upgraded on a flight in 1999 from Portland, Oregon to Los Angeles, from Economy to First - my first time in something better than economy. The story to go with it was myself and the President of the association I was running at the time, got to the airport in Portland two hours prior to the flight, knowing US flights get overbooked, only to discover it was the day the college students were travelling back to college - we should have been there four hours prior at least. We were told that the flight was fully booked, but they checked in our bags and said to go to the gate and see what happened. This was President Keith's first overseas trip and he had been jumpy the whole time, but was now extra jumpy. I wasn't too worried as there was a six hour layover at LAX before our flight back to Aus, but then we found out at the gate that all remaining flights for the day were also fully booked. Time to start thinking about a Plan B. But AA came through for us. They had a boarding pass for President Keith but not for me and he started making a fuss - but while we were standing at the counter I saw a boarding pass being printed out with my name on it, for row 2. I told President Keith to be quiet, said thank you and headed off to sit in the biggest seat I had ever been in, with fantastic service. It was a pity it was a day flight.
Upgrade number two was with MrsK on a trip to the US. We flew into Los Angeles and stayed up in West Hollywood. The plan was after a few days, we would pick up a convertible and drive the old coast road to San Francisco. As we were on a budget, I had booked a Chrysler 200 (I know - like saying a convertible Camry)
We walked into the Hertz yard in West Hollywood and amongst the convertible Mustangs and Camaros, was the lonely Chrysler 200 sitting in the corner - I think they were hiding it. The agent came out and we had a chat about the other good looking cars and she said it was only a "little bit more" to hire a Mustang, so off we went into the office to see what that involved - which was doubling the price, so we resigned ourselves to the Chrysler. While the agent was chatting to us, she said how she loved Australians, and swung open the door to an inner office, where the filing cabinet was covered in Australian magnets, small stuffed koalas on top. After we said we would stick with the Chrysler, she tapped away on the computer, paused while checking something, called someone on the phone but didn't get an answer, so she smiled and said "I have to make and executive decisions - congratulations, you have a Mustang". And we did