For me it would be to gain certain superpowers, assuming that one wish can only grant a single power.
To be able to control the weather, like Storm of the X-Men. This is a way of getting multiple powers for one wish, since weather control allows you to fly on the winds, defend yourself with lightning bolts or hurricane-force wind blasts, and help mitigate the effects of weather related disasters.
To be able to phase through solid objects, like the X-Men’s Kitty Pryde or the Vision of the Avengers. This also has multiple applications, being able to sneak into any secure location (I am insatiably curious about so many things!), pass through annoying obstacles (such as crowds of people) and is the ultimate defensive ability.
To be able to heal oneself and others from serious injuries or other ailments, including infectious diseases or cancers or genetic disorders. I’ve lost too many close relatives and friends to these over the years, and it would be wonderful to alleviate pain and suffering.