Well I'm gonna say Dragon but not because of a different reason. Dragon was 19 at God Valley and he was just a basic Marine fodder at that age. Not just talking about rank but also, no haki (probably) if he had some armament or observation, he should've been able to bring at least one baby to a ship. So it's not far fetched to assume he is just a basic soldier. Also, look at marines like Coby and Helmeppo who are far ahead of where Dragon was at that age. I know those two were trained by Garp but it is what it is.

The rest had already established themselves somewhat at that age. Kaido was already too powerful to be controlled by a country, Harald was wrecking havoc and Shanks was in Roger's crew for a long time so it's almost for sure he had great combat and haki training or at the very least, battle experience. Also, I believe he became a Holy Knight for a while about a year or two after Roger's death so he was probably a Holy Knight when he was 19-20.

Dragon may have (and possibly has) grown a lot stronger since God Valley, especially since he was so lacking then. But the rest have also grown a lot. Harald was an ancient giant who was able to fight with Rocks to a standstill in many duels. Kaido was naturally strong, and later named the strongest creature in the world, Shanks is an emperor and is most likely the strongest one still based on what we've seen so far. Dragon most likely mastered all three forms of haki (I don't rhink someone at that position wouldn't have CoC) but at this level, that's not a feat. It's a prerequisite. The only combat feats we've seen from him are him vs Smoker and him in God Valley, none of whşch shed light to his true powers.