I can't tell you whether or not they are "useful" but I've mostly seen Acid Armor, Counter, Curse and Iron Tail (even though it's now an ORAS tutor) on Goomy. In fact, I'm currently in the middle of breeding Goomy, myself - Acid Armor, Counter and Curse on all three, along with the three remaining egg moves in the fourth slot.Any useful egg moves to put on a Modest Goodra? I'm thinking not, but was wanting other people's opinions?
Anyway, if you're just trying to get the minimum amount of "useful" egg moves, then Counter should be your choice. All others are either useless, tutors, or niche.