Thanks. Zip-ties have been added, grateful for the reminder. My mirror assertion has been rephrased to acknowledge how personal is the choice. Right now, I have a Take-a-Look on my helmet and the Ortlieb at the very end of my drop, with both mirrors set vertically. For years, I went without any.
Your question, Tony, reminds me of the Robinson Crusoe question popular among academics: "Of all books, which would you take with you if you knew you'd be left on a deserted island?" In fact, I am facing just this scenario both with books and with bicycles, as I am to enter service in the Peace Corps.
Perhaps the expected choice, I have to say the Brooks. Whether or not I ship a bike out (and one will be issued), the Brooks will be in the luggage. I can ride anything that's given me contentedly and for a long time, as long as that's my saddle. Were it not for my B17 Narrow, I'd no longer be cycling.
However, I will also be taking with me a pair of friction shifters with Paul's thumbies (mountain bike issued), should I not be shipping out an LHT. Those would be Shimanos for unquestionable simplicity and reliability, leaving Silver/Dia Compes Stateside.
Lastly, Lake shoes with rubber grips and dual-platform pedals might be included, if there's room, or at least Gripsters (never liked clips). The Fiberfix, zips, and a replacement derailleur hanger will certainly make it along with other items of minute size and weight, such as quality levers. The trusty Lezyne floor-drive pump will more than likely make it, especially given its feather weight.
But to pick one--the Brooks. And to pick two, the friction shifters.