Depend on your playstyle. I'll list some ninjas that I believe can be a decent P1 in good to meh order.

1. Sage of Sixth Paths Naruto : This guy is just great, he is the most consistent ninja that can do mystery every round thank to his standard attack. His mystery damage is not bad and he can leech.

2. Angel of God Konan : She is a great nuker if you stack her enough. She also scale based on chakra, super armor and has mystery reset mechanic but not as consistent as Naruto. The reason I place her above Jonin Minato is because she is better than him in PVE activities like exam, sa, ti, etc... thank to her AOE mystery.

3. Jonin Minato : He is the most F2P friendly P1 ninjas around here. He is cheap, his bond mystery actually not that hard to obtain and it's not that expensive. You will need to max assist link for other P1 ninja anyway but with him you get the extra bonus for his mystery. He also scale but with a slower pace than Konan since he scale with his chases instead of standard attack or chakra.

4. Edo Tensei Hiruzen : If you stack him enough, he will end the fight before it even begin. But like I said, only if you stack his power enough to do that. He also very useful in PVE activities or group PVP.

5. Hokage 4th Minato : He can scale and his passive hit like a truck. However he is heavily rely on tag for his mystery to work at it full potential, he also scale pretty slow and don't have mystery reset mechanic.

6. Edo Tensei Minato : He is immune and can scale, his biggest advantage is that his mystery can't be miss and has interruption. But the downside is his mystery has 3 rounds cooldown and no mystery reset mechanic.

7. Shisui Uchiha : He is a barrier ninja, his standard hit like a truck. However he has hard cc on hit effect built in with his mystery that usually get reflect back at you if you don't have mirror or get overdrive by other ninjas chases in your lineup. Also his mystery damage is considered low and has 3 rounds cooldown without mystery reset mechanic. Barriers are easily to counter nowaday is something you want to take into consider.

Pick your poison and don't forget to take other support ninjas into consider as those ninjas above also require some expensive support ninjas to work with them.