I think for grouping content the best choise is folder. It’s clear and simple + you can select all descendants.
Empty Actor is also a very usefull tool when you need to stick a group of object to one origin. Probably gonna use it in future. It could also be used like a sort of inheritance if i got it right.
I don’t like grouping, cause it creates a new object in world outliner, you need to find it, and it’s not that clear which element it’s controlling. Probably won’t use it.
Tag is usefull for some category markup like these guys are fast, those are powerfull etc). And of course layers are super usefull when you don’t want to work and observe the entire level.