My favourite for a few years now is a commercial font, Triplicate. It’s a true serif monospace font; every other monospace font I’ve encountered that is labelled “serif” is actually a slab serif. It has a proper italic face as well, which is a less rare, but it’s a functional italic face rather than ornamental.
It comes with some interesting variants, most notably Poly which is not fully monospaced, adjusting the widths of characters like i to be a little narrower, and W to be a little wider. I like to use that for code display.