Apeks DST (DIN, 5th port upgrade)
Apeks XTX50 (both black, not that yellow silliness)
My reasoning:
-If I only get one set, it needs to do everything that I want it to do. Which means it needs to be compatible with cold and dirty water.
-They have a long track record of being solid regs.
-For all my other needs I just get more of the same. Diving doubles? Just add another 1st stage. Build a stage or want to sidemount? That swivel is awful nice, and it will be identical to all my other regs.
-They are easy to work on, and require a minimum of specialty tools, and being dry sealed will go a long time between needing rebuilds in the first place.
-Rebuild kits a fairly easy to come by, and cover almost all Apeks 1st or 2nd stages depending on the kit, making logistics quite simple. (exception of the flight, but I prefer to pretend that reg doesn't exist...)