Fu dont have BT right now. Ao and Windblade Asuma is bad to get with moon scrolls. If you are F2P you have to spend all your Moons Scroll on Power. Haku falls too early.

Tenten and Sai only usefull to grind. I have Seigetsu full breakthrough but he's also situational.

I have Second mizukage lvl 4 with almost full breakthrough. But now im sure you almost never get fragments on events. So i would not invest in him now.

I have Chojuro lvl4 full breakthrough and he is very fun. if you are lucky to get on Kage treasure he's easy to grind if you can focus on sun scroll events.

Dont forget Water main second breakthrouh is Lightning. So now im looking for Ay edo tensei just 210 seal scrolls on treasure. And you can get 1-2 every day on event. So lvl4 is not impossible in half year. His chase with breakthrough is very strong. So he is the best final F2P ninja. But if you want to go easy sasuke 5 star is a good option. Just maybe too hard to get those Y breakthroughs. but if you are lucky to get the best breakthrouhs he must be good for F2P players.