Forever as a 19 or 20-year-old would be good. You can blend in as a student (high school or college if you want) or pass for 21+ and get into bars, etc.
If I had to choose a power, I would like the power of suggestion. I don't like the idea of fully controlling minds, but if I could easily persuade people, I'd be ok with that. Or, like Bella, I'd like to be a shield and be able to protect those I care about. (Cheesy, I know
.)