Has to be orange of course, a good quality Seville orange is best, with lots of rind as that creates the bite. It also doesn't work unless the toast has lots of butter, they work together.
My breakfast is different but still has marmalade, proper Jewish Beigel from Brick Lane, butter, panchetta (by far the best tasting bacon), a sprinkle of wine vinegar, and then spread thinly with marmalade - ooooh morning heaven with a big cup of Masala tea.