what is the best way to highlight an (STL) object

changing color and opacity of course is possible, but perhaps not the best options. Adding a red outline could also overlap with other objects.

is there any way to add light glowing effect, or change the surface material of the STL object so it looks distinctly different? Thanks for your inputs.

You can play with ambient, diffuse and specular properties of the actor, but that might not be visual enough.
You could also “show edges”, but it might show more triangles than you would expect.
To play with surface materials, you need to activate PhysicalBasedRendering (WebGPU only), but I doubt that it will have much effect.
Depending of your application, you could add a bounding box around your actor, or changing the color of it.
You can get fancy if you start playing with GPU shaders though.