It comes down to a few ninjas, I can't name just 1.

Sage Naruto, overall a great ninja. He can be a Tank, a nuke and fits in a lot of teams so easily then generates extra chakra so *y.

Yugito, a ninja who can revive is always welcome, yeah it *s she is a 2 star ninja when you pull her, but her chase 20 when combined with water main is free poison tai. Her mystery also able to cause ignite really fun ninja to use.

Fu 7 Tails Jinchuriki, she works so well with Gaara (Normal) letting you set up barrier and attack, able to either CC or inflict aoe damage that causes ignite. She also helps sage naruto if his clone dies in round 1 to set up clone then attack.

There are others but these 3 are my favorites.