best and quickest place is a depotplace or rotate them with teaterboxes is also a option
The manual states the best place is NSS (Moscow/Berlin being the two obvious + there are others) Next, is a non-Theater box, known as reserve box, will get allocation of replacements before on map units. Next is a depot, with enough supply, with any HQ with the unit trying to refit on "refit". Next is have the unit on a depot with a HQ within range, on refit. etc etc on down the line. Here is the manual rule for this;
4.10.14. Improving the chances of gaining
replacements and supply
For the unit to get replacements, it must pass both
administrative and support checks and it helps if it is in
refit mode or is located in the National Reserve (13.2).
Units have up to five chances each turn to receive supply
and replacements. Thus if they pass every leadership test,
and are close enough to a depot with sufficient capacity,
they can gain up to 100% of their need.
If the unit is in refit mode it is more likely to receive
up to 40% of the need each time it passes one of these
tests (if the supply is available). Units in refit on any depot
will attempt to get to 100% TOE irrespective of the supply
priority, drawing from other depots as well.
Adding a HQ counter to a depot will substantially assist
any combat units stacked in the hex and looking to take on
reinforcements. Similarly you can leave your rail repair HQs
on a depot to raise its priority for supply deliveries (25.7.9).
Units in refit located in the same hex as a national
supply source depot (type 4) will have access to virtually
Units set to refit and in the relevant national reserve
(13.2) will be the priority for new or replacement
equipment. As far as possible, their needs will be met
before any on map refit takes place.