Power-wise? Time manipulation. Or just manipulation of my perception of time, if that's not possible. As long as I need to think about any problem, the ability to fast-forward through the boring parts of life, reading anything instantaneously, having perfect reflexes because I can just slow the action down to a more manageable speed... it avoids all the usual problems of superpowers by not giving me any abilities that are technically beyond human capability, just makes my mind faster than anyone else's. Or if I get to distort the flow of time itself, it rolls teleportation and super-speed and invisibility and all sorts of other cool powers into one.
Tech? An Omni-tool would be magnificent, between the supercomputer on my wrist, the the microfabber that can break down almost anything into gel and then use that to construct whatever I choose in minutes...
Or maybe a neural computer, allowing me to access all the resources of the internet with a mere thought, perform instantaneous computations with mental commands, and record virtually anything I see, hear, or think.
And of course a sonic screwdriver would be nice. I'd want a variant that worked on wood though; that oversight would just get embarrassing after a while.
Character? First impulse would be Tali. But then I remembered that there's no food for her on earth, or any other planet we have access to at this time. Garrus has the same problem. And once past my first impulse, I had many problems. Twilight Sparkle or Fluttershy? Mordin or ED-E? Simone the Digger or Agatha Heterodyne? Too many choices, and since they're all options as soon as I picked one I'd feel terrible for missing my chance to meet the others. Not to mention the moral implications if I'd whisked them away from their world and into ours without their knowledge or consent. Better to stay away from this category.
I thought about a fully equipped SR2, but a) that seems like cheating and b) there's no way we could reverse engineer that tech.
Meanwhile, I'd stay away from true timetravel, just because I know that I'd try a paradox for the lulz and end up unraveling the universe.
And immortality... I'd love to live for a few centuries in good health. Even millenia. But I want a way out; I cannot comprehend eternity, and there'd probably come an aeon that I got bored and either wanted to die or went truly and utterly insane.