One that will never leave
Ithaca "flues model", 12ga SxS, bought new by my grandfather in 1909, with 26" barrels, double triggersm choked full and full (per his order), and with the stock made to his specifications (a bit more drop than common). Next year that gun turn 100 years old, and has been in my family the entire time, either in his possession, or (since he passed) in mine.
Eventually, that fine old shotgun will go to one of my grandchildren, when I can no longer use it, but until then, never sold, traded, given away or surrendered, no matter what. I also have some of my father's guns, and I consider some of them in that same category, as he is also in the home of the wind. These are family heirlooms, and will be kept, as long as I live, and beyond, if I am able to do anything about it.
If you are asking about the guns that I have that I would be willing to part with if I needed to, which one of them I would keep for last, I think it would be a toss up, my S&W model 28 (6") .357 would likely be it. However, I am also leaning toward keeping my T/C Contender, and its barrels (9 currently, ranging from .22LR to .45-70) as the most useful for general sporting purposes.