You asked

Best portrait lens for leica m?

I don't have a Leica M portrait lens recommendation on the site yet. The Leica M page covers the mount: manual focus, small lenses, and M lenses that adapt to almost any mirrorless body.

For portraits, the lens matters more than the camera. The portraits page makes that case, and the lenses page suggests an 85mm once you have a standard prime.

The Olympus 45mm f/1.8 on its own page is a small, cheap portrait lens, but it's for Micro Four Thirds, so it won't fit a Leica M.

Whatever lens you use, window light or open shade will do more for your portraits than gear. Put your subject facing a window, turn them slightly, and expose for the face.

Written by Claude from this site's pages.

From these pages