It will depend on the listener. Some people can’t focus with certain types of music/instruments. Are you asking for your own practice, or to make music for others?
The safest bet is to find a background sound (almost like white noise), typically a pad or synth with soft melodic sounds (ambient music). You can go deeper and find binaural beats or specific frequencies.
There are lots of great examples for music in general in the Magick of Music Weekly Challenge