Reyna is extremely talented at rallying her troops and encouraging them to do well. It's one of her strengths, after all. Jason comes close, but he simply doesn't have the same amount of talent at the level that Reyna does, plus he sort of gave up his title as a strong Roman leader when he switched places with Percy. Percy may be a good fighter, but he's not the strongest leader. He's rather impulsive due to his ADHD nature and immediately acts upon his thoughts. Hazel is a little tentative when it comes to leading, so she wouldn't be the best choice. Frank isn't terrible, he's getting used to being a leader, but once again he isn't on the same level as Reyna. And we all know that Piper isn't a very strong leader. So I say, definitely Reyna.