GZDoom with low resolution and software rendering, but added light effects.
Doesn't this look fantastic? The light effects blend in so well. Notice the reflection on the medikit.
I prefer playing with a low resolution because it's a more coherent look. Texture resolution, sprite resolution and geometry resolution line up nicely this way.
Also I love how the darker wall at the center is a bit purpleish. That is I think due to Doom's color maps, which often don't have a "linear" progression. Some pixels change their hue as they get darker, which can have some unique and great looking effects that are, I guess, more deliberate than calculated lighting in newer games.