Damn, I was really hoping I'd be the first to say "the power to fill things" :< Given, it's probably well-known by now.
I'd like to have probability manipulation. Think about it: I can increase the chances of my finding $1 million in the next minute, I can decrease my chances of dying from an illness, I can make someone near me spontaneously combust in a heartbeat, and so forth~ Heck, I think I'd even be able to reduce the probability of me dying for the next 100 years and increase my chances of acquiring more superpowers.
If I have to go with a power that's less powerful, it'd have to be the ability to alter or erase peoples' memories. Sure, there'd be limits to it--I wouldn't be able to mess with the memories of someone I'm talking to online, and people with photographic memories probably wouldn't be affected--but it'd help with getting a job, making friends, etc. It'd also help people forget anything stupid I end up doing or saying.