OK, Maxwell remains my favorite.
But, for those that might be interested, the current benchmark for "most technically advanced rendering engine on the planet" probably is Maverick Render. This is the one to compare against. (In response to Unreal, Unity, Lumion and Twinmotion comments.)
Maverick is an entirely GPU-based renderer. CPU is not an option. It works only with nVidia cards, and wants an RTX or greater. And it will only work on a Windows OS. It is a subscription-based application, and is reasonably priced in a Studio and an Indie version.
Having listed those limitations, physically-based lighting and ray-tracing are default features, with a lot of controls over those settings.
Material Editor - Maverick is the only rendering engine that can directly import a Substance archived material (with all its user settings) to use as a material. This means that if you have a Substance, you can apply it to a portion of a model, and tweak its parameters "live" - that is, while in the rendering engine. It has a pretty sophisticated node-based Material Editor that is well thought-out in terms of actual operation. Not only can you make transparent, semi-transparent, subsurfaced, two-side and emission materials with ease, you can start with an existing Substance, if you like, and then add to it or edit it to create even more properties. Maverick does come with a bunch of pre-made materials in its Library (about the same number of and types as Keyshot Library), but the existing Library is not its strength - the Material Editor is. Having used it now for about a month, I can say that I can create somewhat more complex and sophisticated materials with Maverick than I can with Maxwell - but these two renderers definately are more advanced than Redshift, Octane, Corona, Arnold and all the others I tried while preparing the Substance Designer tutorials for glass material.
Lighting - Interestingly, there seem to be two built-in kinds of lighting. In general, you always start with HDRI. Quite a lot of facilities here for advanced work with HDRI's.
But the built-in studio lighting is a "Keyshot Killer" and a "HDRI Light Studio Killer", for sure! To look at the studio lighting tools, here is a video tutorial worth examining.
https://www.youtube.com/watch?v=LjpGAbD ... Q&index=81
What you'll see is this video is the standard method of adding studio lights, an alternate method which targets the objects using the camera's viewpoint, and an absolutely fantastic method that lets you select a focus area on the object itself ("face normals method").
Watch through to the end of the video for this last one - because they saved the best for the last in the video. That is, you select an area of the model to be lit, click on the area, click on a light, and then slide the light over the surface of the model to adjust it to get exactly the effect you want. Of course, you can do this with multiple lights, and adjust the properties of those lights. Further, there are specific mouse controls, so you can position the light reflection, change its size, shape, color and intensity (plus soften the edges), all with one hand on the mouse. Basically, "painting with lights".
And then there is the "Light Mixer" facility.... Like I said, a Keyshot/HDRI Studio Light Killer!
Rendering - A pretty good renderer. Has decent compositing and tone-mapping tools. For noise reduction, there is something called a "HotPixel Removal" tool. I had occassion to use this because I rendered out an architectural scene with a body of water in the front, lots of glass objects and some light emitters. Scattered fireflys all over the place, and I had to ask for help. The Support Staff kindly pointed me to the Hotpix Tool, without making fun of me at all! If you get a render with fireflies, you just decide how you want those "hot pixels" to be selected and then re-render.
I can no longer comment usefully on rendering speed because I built myself a new machine with an nVidia RTX 3080 12 GB card in it. All I can tell you is that where I rendered five different scenes in Maxwell CPU on my old nVidia GTX 1080 card, and they required an hour and half apiece for a 1600x1600 render, ... I can now render those same scenes in 8 minutes apiece. So, I don't really know how much faster Maverick is than Maxwell - if the video card were to be held constant. (I can tell you that I can no longer set up a render in Maxwell, and go make myself a cup of coffee from scratch, and then return to the computer. Damn! Maverick requires a whole life-style change!
So, here are a few reasons I'll continue to use Maxwell and probably work with both applications about eqully frequently from now on.
Maxwell, for me, is easier to use with my large outdoor scenes and for complex arrays of objects. (Which is mostly what I model.) I've used Lumion for years, but Maxwell beats it for quality. Maverick seems more designed for product visualization, than for architectural rendering. Although, I've had good luck with it so far, for architectural work, and have some nice Maverick renders to show for it, Maxwell is just easier and more intuitive for this purpose.
For plants with two-sided leaves, stamins and other plant parts, Maxwell's materials seem to display more acccurately. Same for objects with a lot of sub-surface scattering materials. It is frankly easier to construct these materials in Maverick than in Maxwell, but Maxwell does a better job of making a physically accurate render of these kinds of materials.
There are times when I really need the geographical coordinates. Probably most of you never use these, but my particular kind of work is greatly facilitated by lighting based on earth coordinates and time specifications. Maxwell is the only rendering engine with these facilities.
I haven't really figured out how to regularly get atmospheric moisture working in Maverick. Like I said, Maverick seems focused on studio lighting more than anything else.
So, if you don't like node-based material editing, don't go to Maverick. Some here have pointed out that they never have time to make materials. Maverick will not meet your needs. (But also, don't go to Substance Designer either, and maybe plan on having to outsource your material development needs to specialists increasingly often in the future.) If you're afraid to invest in learning new stuff, or you don't have the time to invest in self-education, don't go to Maverick. This is the absolute bleeding edge of rendering technology. Substantially new capabilities require new ways of thinking and working.
For anyone who wants to take a look at Maverick out of technical curiosity - without watching the usual shallow hype-filled adverts, there is a long series of video tutorials here. https://www.youtube.com/maverickrender
The tuts are numbered in order, so you should look for and start with the first one. But, also look way down the list because as changes or replacements are made to the software, the staff just add tutorials. So, some later tuts will supersede some of the earlier ones. (And, by the way, there is no written documentation for Maverick - there are only the tutorials. Maxwell is much better is this regard.)
Anyhow, just wanted to mention ...