Best way to get 12 V from 28 V or 5 V ?

Gotta be careful with the darlington's tho - they have a higher Vce than discrete NPNs.
The lowest I see at digikey is 0.9V, and that's for Ice of just 300mA.

LM7812 good for up to 35V, 12V output,

will need some heatsinking tho for the 2.4W that it is dissipating.