The water sprite summon copies only operator stats, range, and ATK interval, not trait, so you want to copy operators that have good stats and don't rely on their trait to be good. For example:
Splash Casters -> Meh. They have good base stats, but very low range and a long ATK interval, and it is their Trait that provides them with their AOE so Mumu's Water Sprite won't even get that.
Marksmen -> Decently good. Low base ATK means they don't really do well vs any enemy with decent DEF but they have long range, and a very low ATK interval.
Dorothy -> The upgraded version of using Marksmen. Quite good. Her ATK stat is higher than them, has the same range, and Trapmasters actually have /faster/ ASPD than Marksmen, hitting every 0.85s vs a Marksman's 1s.
Mystic Casters -> Very good. Although their ATK interval is long, they have decent range and MASSIVE base ATK, which the water sprite clones. Ebenholz is an amazing candidate.
Deadeyes -> Also very good. Huge range, huge ATK stats, if you clone Fartooth and multiply her by 4 you can own half the map.
You can apply the same logic for melee units, though obviously you're no longer thinking about range. Don't use anybody with puny ATK that rely on their trait like Liberators. Think big, big stat sticks, like Dreadnoughts!