Deku is definitely one of the most popular. Fans like to explore how his character would develop if he were female, especially considering his relationship with other characters like All Might and his classmates. Bakugo is also a common choice. Imagining him as a girl could completely change his hot - headed and aggressive personality, or at least how others perceive it. Ochaco is another popular character for gender - swapping. Her sweet and kind personality as a male could bring new dynamics to her relationships with the other characters.
All Might is a very popular character to gender - swap in the fanfiction. As a female, she could be a symbol of female empowerment in the hero world. Her relationship with Deku could take on a different, more motherly or sisterly role perhaps. Todoroki is also often gender - swapped. A female Todoroki could deal with her family's issues in a different way, and her dual - quirk situation might be perceived differently in terms of how she uses it. And of course, Momo. If she were male, his intelligence and quirk usage would be interesting to explore, especially in relation to how he would interact with the female characters in a different way.
Kirishima is quite popular for gender - swapping. If he were a girl, her loyalty and the way she forms bonds with others could be explored in a new light. Also, her physical appearance and how she presents herself as a female with a hardening quirk could be really fascinating. Another popular one is Tsuyu. As a male, his calm and collected nature could be emphasized more, and his role in the group could be different. And Jiro is also a character that many like to gender - swap. A male Jiro could have a different style of using his sound - related quirk and his relationships with the other characters, especially the female ones, would be different.