[Suhiir]

As a former Forward Observer myself (when I was a Sgt (E5)) one thing this doesn't take into account is the location, in terms of where on the battlefield the observer is.

It's all well and fine to have a senior FO able to call in and direct the massed artillery of a brigade/division/corps down on an AREA it's another matter when a line company needs a POINT target (i.e. a an MG nest, pillbox, enemy infantry platoon) delt with.

During WW2 USMC moved FOs down the chain of command to where they are now. Company level. YES there is a Battalion/Brigade/Division Fire Direction Center/Coordinator for the (relatively few) times massed artillery needs to be coordinated.

Aircraft/naval guns/artillery/mortars are ALL just methods of delivering supporting fire and treated essentially the same (yes USMC air support is MUCH more closely integrated with the ground elements then are those of any other "Air Force" I'm aware of.)

So "best" is VERY much a matter of perspective.
"Best" calling in/coordinating massed fires?
"Best" at responding to the battlefield situation as it develops?