Well, I think AWC is a strong contender for first on legendary; because recovery times tend to be 30-45 days. On Commander, I'd go GTS no question. My build order for commander is GTS/Power/Workshop. The workshop though is built BELOW power, on the second level (I know you don't need power second, but putting it in the middle improves the flexibility of the workshop and where build times are so short on Commander, it greatly improves the flow going into the early mid game). I then use the Gremlins to dig out the adjacent space, and build Comms, while I build a proving grounds on the upper floor (This allows me to have a workshop adjacent to power/comms, two buildings I know I'm going to need 2 engineers a piece for. It's a mild slow down in weeks one or two, but it speeds up progression for following weeks since the gremlins increase the speed of debris removal for the other second floor rooms and build speed.) So design on commander ended up being.
GTS/Power/PG
Comms/WS/AWC
For Legendary, I'm almost through, my design was GTS/AWC/Power, then I built the workshop below power, and built comms below the AWC--built the workshop first to use the gremlins to dig out, less flexible design but it let me rush the AWC, and my workshop is still adjacent to power/comms and it can dig out a third floor facility. Proving grounds got pushed back.
GTS/AWC/Power
PG/Comms/WS (Workshop was third build, to speed up excavation/comm building while keeping enough engineers for AWC)
This all really depends though on how lucky you get with engineers in the first two months. My strategy essentially counts on getting dog shit for engineers. Because my legendary start I only got the engineer from the Guerrilla ops mission (It's much easier if you also get a VIP engineer); you generally can't buy an engineer if you want to keep building new facilities and given build times you don't want to delay facilities.
AWC : Power : PG
Power: WS : Coms
GWS : x : x
Why multiple power buildings? Upgrade your power facility and slot an engineer in, and it will give +7 power in total (+2 for the upgrade, +5 for the engineer), you can do this twice. It's why you try to keep power adjacent to the workshop, to let you double up engineers to run it. (By the time you need a second power building, you should be able to built it over a coil.)