Well, the ability that I choose has to be able to make me money in some way; or else what's the point? Assuming I can't reverse engineer whatever it is, I think I would go with the portal gun. We're talking about a device that can instantly link two areas together. We could use that gun to colonize Mars, saving billions of dollars in space shuttle trips.
On the other hand, some kind of healing spell, similar to the Medic in Team Fortress 2, would also be very useful. Imagine, somebody could be in a hostpital dying of cancer, and with a spray of that beam everything is cool.