Out of the main cast of Avatar: The Last Airbender it's not exactly a hot take to call Toph a fan favorite. Without any doubt, Toph deserves a lot of the credit she receives. She was a self-taught earthbending master who learned to use her disability as an advantage, invented the art of metalbending, and is possibly the strongest earthbender of all time.
But comparatively, far too little credit is given is Katara. In fact, Katara may be an all around better bender than Toph.
Our first point of comparison is how each of the heroes first learned to bend. Both were self taught and both ascended to the rank of master early on in their lives, rare feats within the world of Avatar. For Toph, this process of self-teaching involved advancing past a private tutor her parents hired, practicing on her own with the earthbending badgermoles around her home. Toph's tutor babied her on her parents' instruction because of her blindness but that same blindness allowed her to connect with the badgermoles and learn to "feel" the earth around her with her seismic sense.
But that's not quite the same as being completely self-taught, is it? It's impressive, without a doubt, but Toph had the guidance of a tutor to teach her the basics and the original earthbending masters themselves to help prop her up. By contrast, Katara grew up completely without any other waterbenders to even observe. Getting hold of a waterbending scroll aided her in teaching herself, but by the time she first found Master Pakku in the Northern Water Tribe, she was practically a master herself. She even stood up to Pakku in their duel, and he declared her a master not long after beginning his training with her.
It wouldn't be fair to compare how each bender started out in order to gauge where they ended up, however, and when it comes to achievements it's hard to beat Toph's invention of an entirely new bending technique. Metalbending proved to be a world-rocking innovation, with Toph establishing herself as a legendary figure for founding the art and creating the Metalbending Academy and Republic City's police force using the techniques she developed.
While Katara did not develop any techniques as revolutionary as metalbending, she did show her own strengths in innovation and mastery of bending subskills that potentially outweigh Toph's accomplishments. Sweatbending, as gross as it is, was a legitimate innovation on Katara's part. She even discovered the subskill of healing on her own before going on to be declared the world's greatest healer. When exposed to bloodbending by the woman who invented the technique, Katara mastered it better within a matter of moments and overpowered old Hama.
Toph may have invented metalbending but she was never declared the greatest metalbender in the world. According to the feats performed in the course of Avatar and its sequel, The Legend of Korra, it even seems like Kuvira or Toph's own children perform more impressively with metal. Katara was not only the greatest healer in the world but possibly the strongest bloodbender it ever knew -- outside of the genetic anomalies in Yakone's family. In fact, comparing feats performed by the two young benders is probably the best way to see that Katara is the stronger of the two.
Toph was always a contender for being the strongest earthbender of all time, with Bumi serving as her main competition for the title. But in terms of raw strength, Bumi seems to outweigh Toph, with his liberation of Omashu showcasing his ability to toss entire factories into the air and his assault on Ba Sing Se featuring his ability to stack tanks one atop another. Toph just never does anything with such raw strength, with her failed attempts at holding Wan Shi Tong's library serving as an ambiguous indicator of strength far beyond anything else she ever does.
But Katara? She's without equal. Not counting Avatars, Katara's ability to command tidal waves that move battleships or flood factories is unprecedented. She slices through metal and stone, levitates pools of water larger than any other waterbender, and her ability to ride waves of water and slides of ice become so proficient by the time of the comic books that she can practically fly. Pakku may have had the edge on Katara when she was just starting out, but by the time Katara matured there was never a waterbender in either series who came close.
Even if the metric you used to decide the better bender came down to versatility rather than power, Katara would win out. Toph's bending style and seismic sense made her uniquely proficient at fighting other earthbenders; capable of anticipating and countering their attacks. But her ability to anticipate airborne attacks proved limited, and she had few showings against non-earthbenders that would indicate much versatility. Just look at Katara's ability to outfight waterbenders, earthbenders and firebenders alike and her advantage is clear. She even took on Azula twice and came out on top!
Toph may have been more than happy to declare herself the greatest earthbender alive, but sometimes it's the quiet ones you should really pay attention to. The next time you're giving credit to the benders of Avatar just make to raise a glass to the Gaang's waterbender.