The superpower I would choose certainly would be time travel / time manipulation.
The flow of time has something mysterious that has always fascinated me. Being able to slow down time essentially grants immortality, only that nothing really happens in the outside world. Still unlimited time to think and be alone. No more stress, always have all the time you want. Sounds like paradise.
And that is only the beginning. Time travel means you can visit some events multiple times, you can experience the past, you can see the future.
The possible downsides depend on the laws of physics. Can you accidentally create closed time loops or create events that will cause causality to break down and destroy the entire Universe? Maybe?
Might a small change in the past ruin the future and then you create an even bigger mess when trying to fix it?
Probably!
Time manipulation is a really interesting power. I mentioned in another comment, that if you moved in normal time, whilst the rest of the universe was moving at a slower relative time, then you would also be aging faster relative to the rest of the universe! That would mean each time you used the power to slow down the universe, you would be getting older compared to the twin that was in the "normal" flow of time?
I think time travel is really interesting as well, although, I would only do it to view things and inform my curiousity! Like you said, it could be that trying to change something in the past might make a worse problem than the one you tried to fix!
The winners and the new question (I've told you how to do this before...) is here:
https://steemit.com/newbiegames/@bengy/steemitbasicincomegiveawayivetoldyouhowtodothisbefore-pb6n4om11j