Courier New (Visual Studio). I keep trying other stuff, but they are a distraction. Courier New does not draw attention to itself, just seems to blend in, and the code communicates directly to you.
Courier Prime is a very attractive font. Unfortunately the leading (inter-line spacing, pronounced "ledding") is 18% bigger than Courier New. In a full screen of code, I lose 7 lines. That means less visible code, more scrolling up & down. I suppose there are font editors to adjust that.
