Gen 8 Sword - What are the best Egg Moves for Pokemon?

Depends on the Pokémon in question and what you want to do with it afterwards. An Egg Move that is difficult to achieve (because you need to chain breed and/or get earlier Gen Pokémon with said moves as parents) might be highly sought after by breeders even if the move itself is meh. If you plan to use the Pokémon competitively or trade with people interesed in geting their hands on such 'mons, then moves with high attack power, STAB or useful extra effects might be a higher priority even if they're easy to achieve.

If you're just doing it for fun without a specific goal in mind, then just go wild ;) That's what I usually do at least, sometimes breeding EMs solely for my own amusement, like Togepi with "Peck" (can you imagine that flat-faced weirdo pecking someone?) or Treecko with Dragon Breath (not particularily useful but I found it funny that a pure grass-type can learn dragon-type moves)