I don't know if anyone has said his already but i honestly think it would actually be my power. I'm really persuasive and a ridiculously good liar (i know that's quite bad haha) so....
My power would be the ability to make anyone believe what i said without doubtin me at all- they would just have to believe i. I suppose it could be a really evil Volturi-esk power if i told someone they were dead (bit grim i know) and they would completely freak out and stuff. But i could use it nicely, like if someone was doubting themselves, or blaming themselves for something bad that had happened, i could tell them it wasn't their fault- and they'd have to believe me.
Oh and also i just thought if i was in a Bella-Edward situation (think Bella not believing that Edward loved her), then i could tell someone i loved them and they would believe me and not even give it a second thought.
Ok i have clearly thought too much about this