For single illustrations, left top corner unless there is a visible light source. I'm right handed but leaning toward ambidextrous.
But I don't do a lot of single illustrations without clear light sources.
In my comic, I have a lot of exterior scenes, so I put the light source where the sun/moon is supposed to be.
This is theory. I often mess up because it is difficult to remember all the times to take in account seasons, hours, directions etc.
But as a general rule that's what I do.
Indoors I usually have a visible light source (either a lamp/candle or a window).