E flat or D sharp
for notes, you'll want whichever one will allow you to use a unique letter that's different from the notes immediately above and below. so in other words if there is already another type of E above or below, use D#. and if there is already another type of D above or below, use Eb. However, if we're talking about the 'key' of Eb or D#, or the 'chord' of Eb or D#, assuming it's major, then you will definitely want Eb, unless you are in the mood to have double sharps all over the place