A good one is 121/123x7x7.
You can get 1 million regeneration and 1 million storage.
(4,3,4,3,4,3,4 power lines each layer and the core inbetween the bottom one. The rest filled with power tanks or whatever you like).
Because a docking area is 9x9x9 (I guess) you can also add nice hull, more storage (for alternating amc and power drain) or equip shields/AMCs/thrusters to get a ship.
It is not the most efficient (that would be a line), but I guess it is very efficient if you count docking enhancers too, and also very versatile.
I have heard from some 23^3 cube reactor making 1 million.