Minecraft wiki entry for Best_enchantments_guide, associated text: Unbreaking III
Mending
Efficiency V
Sharpness V or Smite V (allows 1-hit killing of most undead mobs) or Bane of Arthropods V (allows 1-hit killing of arthropods) or Cleaving III [upcoming: JE Combat Tests] (extends shield stun duration and increases damage) [note that axes are more useful for combat on Java Edition]
Silk Touch (useful for melons, bookshelves and beehives) or Fortune III (only slightly useful for melons)