Now I've gone MFT, I plan to eventually get a nice lens around the 15-20mm mark as my walk-around lens for travel and home videos. In combination with the ETC mode on the GH5 that makes it a ~35mm lens and ~50mm equivalent lens - the two most flexible focal lengths.
My criteria are:
nice MF ring
aperture control (manual aperture ring or electronic control from the camera are both fine)
faster than 1.8
nice 3D rendering
neutral or on the warmer / nicer /