A VMax or Varmint-type Ballistic Tip that is going to cause as much damage as possible.
I have seen good things of the 58grn VMax and 55 & 70grn Nosler Ballistic Tips already mentioned.
At least then if your shot placement isn't perfect, it is still going to put it down in a hurry. This is obviously better for the fox and makes it easier for you to then find it.
As such bullets will normally be a light for calibre, they also tend to have other benefit of being driven fast and are comparably flat shooting which is helpful.
That said, if I am stalking and a fox appears, I'll use whatever is in the chamber if it presents a shot.