My very first flight in BA First was also the best flight on BA - IAD-LHR in 2006 or so. Only two of us in First. I started with Blue Label on the ground (two glasses that is). For dinner, cabin crew left the Bordeaux bottle on my table - ehmmmm,... I do not quite remember the rest after the rather good steak, but did not feel very well when getting off the plane in London. These were the times when they actually had really good reds in BA First.

But I think the best flights are the ones with the completely unexpected upgrades. LHR-FRA-BOG a few years back on Lufthansa, mileage ticket in Business. Family already in BOG, so flew by myself. As I had to check luggage, did not bother checking in on-line but queued early morning at T2. When check-in agent wanted to print boarding pass FRA-BOG, she could not. When I glanced at her screen, I noticed the seat change to 1A. At the end, she did manage to print boarding pass. Very excited, obviously! One of the nicest flights in a long time.