Lets say it's an generic ninjutsu
Perfect chakra control Sakura, who has knowledge on the body, and the mechanics behind most ninjutsu, gen jutsu and tailjutsu.
Sharrigan Sasuke, who can copy attacks, and read hand seals.
Or Naruto, who has alot of chakra. And great will power.
Lets say its a clone jutsu.
Naruto (after 7 tries)