While the Dragon Ball franchise is primarily known for its extraterrestrial Saiyan characters Goku and Vegeta, several human fighters have also achieved impressive levels of combat strength. Krillin and Tien Shinhan are certainly among the strongest human characters in Akira Toriyama's manga/anime story, but there may be a dark horse candidate who has only scratched the surface of her potential: Videl.
Videl eventually settled down after marrying Gohan as they raise their daughter Pan together. However, she could very well become the strongest human in Universe 7 should she ever resume her training in earnest.
Gohan first met Videl as a high school classmate seven years after he saved the world at the Cell Games. The teenage daughter of Hercule Satan, Earth's presumed savior, Videl strove to be worthy of her identity. By the time of her Dragon Ball Z introduction, she was already a formidable martial artist in her own right, also well-known and respected among the local law enforcement for aiding them in keeping Satan City safe. Videl eventually deduces that Gohan is actually the Great Saiyaman and threatens to expose his secret -- unless he teaches her how to fly.
Gohan observes that Videl is already far stronger than her father and agrees to teach her to fly and master her ki, though the latter technique visibly exhausts her. Videl's newfound skills are put to the test at the 25th World Martial Arts Tournament when she takes on the villainous Spopovich -- secretly upgraded by Babidi's dark magic -- in the first round. While Videl puts up a valiant effort, even accidentally breaking Spopovich's neck, her murderous opponent beats her to within an inch of her life when his unnatural power boost wears her down. Non-canonical adventures have Videl facing off against Broly and hordes of enemies escaped from Hell, outmatched but surviving after putting up a brave fight. By the end of Dragon Ball Z, she developed her own superhero alter ego and continued to fight crime alongside Gohan/Great Saiyaman.
That Videl grew strong enough to learn how to fly and control her ki within a six-month period of dedicated training suggests she is a much faster learner than some of her more established counterparts. Both Goku and Krillin took approximately three years learning how to fly after first witnessing Tien unveil the technique at the 22nd World Martial Arts Tournament. Krillin also took a full year of training under Master Roshi to learn how to perform the Kamehameha Wave. While Videl may have put her training on hold to raise a family with Gohan, she proved herself as a quick study capable of rivaling her fellow human warriors for the title of strongest human on Earth.
Like Krillin and Yamcha, it is unlikely Videl will ever truly rival franchise mainstays like Goku and Vegeta in raw combat power. That said, she has mastered techniques that make her exponentially stronger than most humans and has certainly impressed Gohan and the Z Fighters with her natural aptitude for martial arts. Videl's priorities have changed by Dragon Ball Super, but the crimefighter still has plenty of potential to pick up her old training -- possibly growing stronger than ever before as she takes on more advanced techniques directly from the Z Fighters.