ReAction is the official AmigaOS GUI, this applies to both 3.x and 4.x.
The vast support from third party developers made MUI the
de facto standard Amiga GUI a long time before Haage&Partner and Hyperion gave things a try. This massive support from developers came from MUI being so easy yet so powerful from a developers point of view (which answers the original post in this thread). Go back and look what people have said about it over the years. Why did Stefan Burström choose MUI for IBrowse for example? It has simply been what most developers prefer!
The reason Haage&Partner (and later Hyperion) used something else, was for intellectual property and money reasons. Technology had probably nothing to do with it at all. This is also yet another example of Hyperion's "OS4" product having to settle with the left-over Amiga standards, after MorphOS already secured the best ones.
From a user's POV, MUI is available for
all *miga OS's, and offer great flexibility and level of customization. From a developers POV, MUI is the easiest and most powerful GUI to work with. And don't take my word for it, listen to the testimonies from all the developers who used it during almost two decades.
The choice should be fairly easy!