Theoretically, the OpenGL, WebGL, Vulkan, Metal, and Unity implementations can all have comparable quality with enough dev work. From a quick glance through the README, it doesn't seem like the OpenGL implementation is relying on any cutting-edge features of the specification, so everything can be ported to WebGL as well. If anything, the Vulkan and Metal versions have higher "potential" when compared to older APIs like OpenGL because if you optimize with the lower level primitives, you'll have more cycles for complex rendering techniques. Also, I'd argue that Vulkan can be used by individual developers because there are already a bunch of games that have Vulkan support:
https://en.wikipedia.org/wiki/List_of_games_with_Vulkan_supp...Actually, if continue under this world with unlimited developer resources, the Cycles version should look the most realistic. It's markedly different than any of the other renderers included in this list because it's an offline path tracer whereas Unity and friends are for real-time rendering. We rely on a lot of hacks (Phong shading, FXAA, normal mapping) to produce an interactive 3D scene at 30+ FPS, but once you remove the real-time constraint, we can apply a lot of very fancy algorithms to more realistically render your scene. Cycles might take minutes or even hours to render a single frame, but it spends this time on a whole host of involved computation (physically-based BSDFs, Monte Carlo integration, Metropolis light transport, etc) that more accurately model how light interacts with materials.