I would go with invisibility. Time travel can mess stuff up.
EDIT: I'm glad this is back. Many memories in this thread.
-
Edited by Explodingpiglets: 3/6/2014 2:37:03 AMInvisibility. Even if somebody could travel through time, they have no idea how their actions would effect the future. Though it would be pretty funny to show up at the iPhone reveal with my HTC one. (even if the online features wouldn't work.)