Monkey D. Garp is hailed as the "Hero of the Marines" for what he did at God Valley. Garp allied with Roger to defeat the Rocks Pirates and erase the looming threat against the World Government. Despite being offered many opportunities to be promoted, Garp turned all of them down for the sake of his freedom.
Garp has remained a Vice Admiral for the majority of his life, but no other One Piece Marine has ever surpassed him in terms of strength. During his career, Garp has taken on many students, both Marines and non-Marines. But who is the strongest One Piece character that he has trained?
6 Helmeppo
At one point, Helmeppo was nothing more than the spoiled son of Axe Hand Morgan. He used his father's name to bully other people and take unfair advantage of them. Helmeppo continued his tomfoolery until his father was taken down by Luffy.
Subsequently, Helmeppo joined the Marines and was pitted along with Koby. Monkey D. Garp trained the two recruits personally, and they became a lot stronger. Helmeppo started using kukri knives and gained enough skill to become a Lieutenant Commander. Despite being a scaredy cat, Helmeppo does not shy away from risking his life to save Koby.
5 Koby
When Koby was first introduced in the series, he was nothing more than a timid weakling who couldn't even stand up for himself. Due to his nature, Koby was even scared to tell Alvida that he wanted to leave. Fortunately, he met Luffy and told him about his ambition. Luffy freed Koby from the clutches of Alvida, which gave him another opportunity to live his life.
Shortly after joining the Marines, Garp took Koby under his care. The Hero of the Marines put Koby through strenuous training, which helped him become stronger. Koby's physical abilities increased by several folds, and he eventually rose to the rank of Captain. Koby is nowhere near Garp's other students, but the Vice Admiral still has very high hopes for him.
4 Portgas D. Ace
Portgas D. Ace was the son of Gol D. Roger and Portgas D. Rouge. Before Roger died, he told Garp that he was going to have a kid and wanted to look after him. Garp was shocked at the request, but he did find the child and save him. Garp and Ace spent a lot of time together, and it is obvious that the Marine must have trained him at some point.
Garp wanted Ace to become a Marine, but the latter did not want to be tied down. Ace wanted to grow up and become a Pirate, a dream that he stated multiple times. It often led to him being reprimanded by Garp. Despite the fact that Ace was an infamous pirate, Garp could not bring himself to hurt him. He even refused to fight in the Summit War because of his emotional connection to Ace.
3 Kuzan
Kuzan is currently one of the Ten Titanic Captains of the Blackbeard Pirates. The former Admiral joined the crew after losing the fight against Akainu in Punk Hazard. The two battled for ten days, and eventually Akainu came out on top. Prior to his defection, Kuzan was among the most well-known Marines.
He was considered one of the biggest powerhouses of the Marines, and he played a valuable role in the Summit War. Before becoming an Admiral, Kuzan personally asked Garp to train him, but the latter refused to do so. However, Kuzan wasn't ready to give up, and he kept insisting. Finally, Garp did accept him as a student, and they trained in ridiculous ways. They would punch the hulls of massive battleships without using Haki or any Devil Fruit powers. The training helped Kuzan reach a new level altogether.
2 Monkey D. Luffy
Monkey D. Luffy is the captain of the Straw Hat Pirates and one of the Four Emperors. Ever since he met Shanks, Luffy's mind has been set on becoming the Pirate King. Needless to say, this decision did not go down well with his grandfather, Garp. Even after being scolded, Luffy did not budge and wanted to achieve his dream.
Garp left Luffy in the care of Dadan, as he could not spend a lot of time with him. However, he came by to check on him whenever he could. Garp really believed that maybe Luffy and Ace would become Marines, especially after Dragon's rise to the status of the most wanted man. Sadly, Luffy already knew what he wanted to do, and not even his grandpa could change his mind.
1 Monkey D. Dragon
Monkey D. Dragon is the son of Garp. The Marine presumably had high expectations for his son, and maybe he even wanted him to walk the same path. However, Dragon was not on the same page as his father. It seems that Dragon learned about the true nature of the World Government, so he could not bring himself to work under such an organization. So, Dragon created the Revolutionary Army and sought to change the world.
Considering Dragon was his son, it is indubitable that Garp taught him everything that he knows. It also implies that Dragon is among the strongest characters in One Piece. Whether he has surpassed his father remains to be seen.