Mission: Infiltrate Hydra, claim its secrets, and cut out the heart: Red Skull himself.
Operation Heart Breaker:
Infiltrate the undersea Hydra headquarters in the Pacific, locate the research lab, steal all project notes, and purge the lab. Secondary objective: Kill Red Skull and his head researcher.
I. Shape Shifting (Mystique)
The ability to take any shape (much like the famed Mystique) is invaluable when infiltrating such a closely guarded organization. It's preferable for them to never know they were infiltrated, so the ability to look like any other wandering guard will prevent many conflicts before they start. As well, the chance to turn into Red Skull himself as a last resort could yield fascinating results.
II. Mind Reading (Miss Martian)
The ability to forcefully rip intel from the mind of an enemy (in much the same way Megan did in Young Justice--repeatedly) is essential for what is primarily an intelligence and sabotage mission. It would allow the agent to slip into the role of an eliminated guard seamlessly, as well as detect patrols before they see you.
III. Teleportation (Goku*)
Perhaps the most valuable ability would provide the escape route. Once the requisite intel is obtained and the lab burned (and, Fates willing, Red Skull eliminated) the hardest part would be getting out safely, especially if an alarm was tripped. Thus, teleporting. Already having an exit point in mind and secured, it becomes a simple matter to leave once the job is done.
(I mean, Goku could just blow it all up, but we're not sending Goku. *And before you complain, it only specified they had to be fictional.)