That's a difficult question, as most adjacency bonuses have been removed, having an optimal layout is not that important anymore. It's more a question about WHEN do you build a facility rather than WHERE and this comes down to personal preference and needs. Psi-Lab is something you'll want early, if you want to heavily use the class, but you may wait a long while, if you don't feel they are necessary. AWC is important to get, if you suffer from a lot of wounded soldiers, but doesn't do all that much in the early game otherwise.What I can say, is you want to get the GTS and the Comms quickly on higher difficulties.The only base-placement you have to worry about is the workshop, as it still provides adjancency bonuses, regarding that, please check this question:There may be an optimal placement strategy, if you can build 2 workshops, but that's not possible, I finished my workshop and cannot build another on the tile next to it. I find it's best to optimize placement, instead of rushing certain things like in XCOM 1.Workshop should be placed in the middle, second row down. A lot of the other facilities should be built around the workshop.Your first facility should almost always be the GTS.