In the context of mainstream usage, ImageMagick probably held the swiss-army status for automated/mass image processing and we could argue why not GraphicsMagick instead? ImageMagick has the most complete raster image format support (including good read but mediocre write support for vector image) and the most important is they have detailed yet easy to follow tutorial and manual to let users know why it's a swiss-army tool.
The other two have different niche but capable of doing many of mainstream usage: VIPS and the fairly new OpenImageIO (used by Blender). The first was best know as the specialist for processing large image (large = very very high resolution) without choking your RAM, the latter is currently better known as a library, thanks to its permissive BSD license but I see the potential for its interface to replace ImageMagick because OIIO also a bit faster in conversion task. VIPS has extensive mathematical operation which make it also scientific niche, OIIO in other hand have distinguished texturing features which make it a bit 3D graphics niche, at the moment OIIO CLI tool was rather simplistic but I'm looking forward for big evolution in this area. Both has official python binding support which should be more interesting for advanced users than what their CLI tool offer additionally VIPS has GUI interface called nip2 which IMO may hinder its true ace.
Even so I found none of them have support for other specific task such as RAW image processing or decent vector image conversion which I resolve by using several utilities. In the end I guess there is no such of a real swiss army image tool yet... not at FFmpeg level at least or imaging was just too broad to cover entirely.
What I wish for these tool is a kind of loadable and user-friendly inline/offline FX subsystem ala GIMP-fu/PyGIMP where artists can share/reuse/enhance their template.
Binaries (mingw build):
OpenImageIO 1.0.8: oiiotool.exe (1.7 MB) maketx.exe (1.7MB) since I don't build iv here is a small exrdisplay.exe (300 KB)
VIPS 7.30: vips.exe (2.1 MB)
ImageMagick 6.7.8-6: convert.exe (2.1 MB)
Features enabled as much as possible with exception some ImageMagick's vector support which I found not that good. This builds can be considered as update for my previous ImageMagick build.