Gohan is op.

Even kenpachi begin to understand own faults and develop.

Armin is thinker and he achieved high command position that he deserves.

Rock Lee literally take own potential from zero and elevate it to very respectful highs with help of his teacher and own willpower .

For me, Gohan was very wasted

Because main character not him but his father.

But Gohan can ask Goku to train and achive angel level of power very quickly.

That possible if author want.

None. Lemillion comes back in the manga. Rock Lee already showed it and didn't waste it. Idk who Adam Taurus is. Idk about Gohan as well but he could have become powerful. Armin Arlet also showed his potential same with Kenpachi. I'll go with Gohan

Don't know about the rest, but Rock Lee could've easily been the real main character of Naruto, considering it USED TO BE about hard work, not natural talent. And Lee trained harder than almost any other kid.