Vivi wouldn't be content with just one super power, she'd want to be an evil dark queen, ruler of everything and servant to none, so she'd pick all of them. Buuuut if we limit her to only one, she'd probably choose teleportation so she wouldn't have to walk anywhere like the lazy vee she is.

Stacy would take the ability to breathe underwater to make up for the fact she didn't become a vaporeon.

Rikki would pick time travel, specifically to be able to undo past mistakes.

Alex takes super strength. The less people that are able to stand in her way and the more things she can brute force her way through the better.

Zoe would pick some sort of healing and calming powers to heal others and instantly calm them down when needed.