Oolong famously wasted a world-domination-stopping wish during the Emperor Pilaf arc, asking Shenron for a pair of "hot babe" panties instead. A truly unforgettable, if silly, moment.
Commander Red spent years building the formidable Red Ribbon Army, only to reveal his ultimate goal was a wish from Shenron to simply grow taller, embarrassed by his height. What a letdown!
Sorbet's dangerous wish in Resurrection 'F' brought back the tyrannical Frieza from the dead, unleashing a new wave of terror after his defeat by Future Trunks. A truly catastrophic decision.
King Piccolo's wish for restored youth in the early Dragon Ball series was a terrifying moment, allowing him to regain his full strength and unleash a reign of terror.
Zamasu's ultimate wish for Goku's body using the Super Dragon Balls led to the creation of Goku Black and the catastrophic destruction of an entire timeline. A truly devastating act.