I can't choose one to save my life so I'll have two.
First is the NG United Her Art Here California. Just such a stunning livery that well represents the state I used to live in. I did also see this registration once before it received the new paint job, and once after.
Second is the NG Delta B738 N3734B. On the outside this looks like yet another same old US narrowbody, but it really is not. I flew on this registration on a memorable and crucial trip, and it also happens to be the first (of many) registration requests fulfilled by NG. Cherry on top I was lucky enough to see it once again last month.
Both of these are 2020 releases and are long gone from retailers. If I were to be put in a situation to relinquish my models (which I hope will never happen) these will never leave my hands.
It would be a very difficult choice but would end up in the end keeping my Lufthansa 747-400 in the old livery as it is a very sentimental model to me.
However if it were to include my none 1:400 I think I would save my westjet 737 from daron. I have had that model as long as I can remember and it was my first ever one.
No contest - Aeroclassics BEA red square Vickers Vanguard. I fell in love with the type ever since seeing one for the first time at Farnborough in 1959, then spotting the whole fleet at Heathrow throughout the 1960s. If AC made more Vanguards in any livery, I would buy every one! Perfect propliner model!