Optimally *nothing*. Silence is nice.
But there are some types of music which make working at least more possible than some noise from neighbors or outdoors.
One important factor would be, no vocals, or in a language I do not understand (wouldbe english grunt vowels of death metal counts here )
Vocals tend to be too distracting.
- some types of instrumental jazz
- some "ambient" stuff, possibly movie soundtracks like alien. Makes somewhat nervous and works comparable to caffeine in that regard ^^
- some classical music, e.g. baroque
- metal with non-recognizeable or no vocals, esp. jazz metal, math metal - it might seem very distracting if not used to it