Sadowsky flats or DR flats - both are much more forgiving and flexible than most flats, and have plenty of life. Black tape wounds can be nice, but they also break the earth connection at the bridge so can make your instrument a bit buzzy (not in a good way) in some venues.
My favourite for fretless is still DR Sunbeams (or Pure Blues). Nickel round wounds, but gentle on fingerboards and it takes a long time before the marks are significant, even on a rosewood board.