Best reflection method

There is also screen space reflections, which are really just for the aesthetic of reflections, but cannot reflect anything that isn't visible in the view.

Yeah I think one of the most common solutions on this generation of games will be to use a lot of cube-map "probes" around the level, but combine those results with screen-space reflections (ray-tracing through the depth buffer) to add in self-reflection and other local details.

In the future, we'll probably see ray-tracing through voxel representations of the scene become popular (partially resident sparse voxel octrees, etc).

If your reflective object happens to be a flat plane, then old-school planar reflections are tried and tested there :)

Technically we don't necessarily need voxel representation of the scene, you can use standard triangular representation of the scene, good quality BVH and perform ray tracing (assuming you do deferred rendering, you really just need to perform ray tracing of secondary rays). This way you achieve good quality hard reflections (of course you can change them into smooth reflections - either by using some "smart" blurring (you need to take edges into account ofc) or "use more rays").

if the reflected scene does not move, you can prerender the cube, and after cube is rendered over the surface of reflecting object render additional dynamic possible aproachers, that will be depth tested only with each other if there are more than one (cube should be always occluded- it can bring an artefact - that will very rarely- impossibly perform though).

Though this may sound as a radical optimization, it is still an untrivial operation. Though you can make cube texture of very friendly dimensions (256x256) .

But as you appointed, cubes can fit only some unimpressive scenarios (metalic pen?).

If you wanted to have a big reflective wall, and player looking at a small portion of the wall that covers his entire screen, cube maps are useless.

Robust reflection can be achieved only by rendering mirrored (repositioned) object of scene onto observing render target once again.

If you are facing a reflector that is not of planar surface, you might try mirroring every object of scene on each triangle plane, but before issuing rendering, check the AABB of mirrored object covering the triangle area (done on CPU trivialy)- this could bring a pretty acceptable aproach as you would filter quite a bunch of renderables in my eyes, making a quite equal fillrate over reflecting surface.

this could bring a pretty acceptable aproach as you would filter quite a bunch of renderables in my eyes, making a quite equal fillrate over reflecting surface.

though! I am not realy encouraging you to perform resolved reflection on a unplanar surface (you should not do it- you still can do planar reflection of observation in world, and refract unaccurately (but exactly) on a shaped reflector).

I stress that reflecting to observed scene is far from trivial- from computaion resources especialy - it is actualy dislaying scene you do not look at- in scene you look at