I'd say Chi-Chi, Videl and Pan.
They have 3 things in common: They're girls, they are/were fighters and they had great potential.
First of all, Chi-Chi was a martial arts prodigy, even able to rival Goku as a child and pressure him as a teen. She was tough and an absolute badass. But she ended up as a housewife and a terrible, close minded mother who stays on the sidelines.
Then we have Videl who got the same treatment as Chi-Chi but worse. Chi-Chi at least kept her courage and tough personality. But Videl's personality did a 180, making her bland and wimpy.
And at last, Pan was a quarter saiyan and a powerful fighter who absolutely crushed a huge guy at a martial arts tournament when she was only 4! But then during GT, she became a damsel in distress with her main thing being getting saved by Goku. And she didn't even go super saiyan!