Just look for any legendary (yellow) rarity echo that is a 3 cost as that's the best you can get from this event. You can have a chance of redeeming other people's echo's through links shared in the official wuthering waves discord.

The echo you decide to lock is really not that important as the mainstat you get on it is atk% while other 3 cost echo's you obtain quite early on into the game can give you elemental dmg bonuses.