The most feature rich browser engine (Chromium) is the one with most adaptation across all the most used platforms so that doesn't make sense unless you count users from something like AmigaOS and TVs and in that way get Firefox out on top by purely counting platforms. Otherwise Chromium on Windows, Mac and Linux beats anyone else by a mile.
Even for platforms that Chrome can't easily be ported to, WebKit itself has a very simple and actually reasonable porting API which led to it for example being used on Sony PlayStation platforms, but also being the most important browser engine on the Haiku OS.
In fact, Chrome originated as - and a lot of its code structure post-Blink-fork still signifies it as being - a meta-platform of its own with a WebKit port to it.
