Portal Gun sounded interesting, until I thought of what it would feel like to have half of my body trapped in a wall, or realized that the only thing that made the Portal Gun somewhat safe was the spring thingies on the girl's feet that let her fall far distances without the side-affect of death.
Cloaking devices are cool but they don't protect you from unaware oncoming traffic; so don't forget to look both ways n' such.
A Gravity Gun would seem to require quite a bit of skill to use effectively and I'm always weary of things that can let me lift things I'm physically incapable of catching should they fall in my direction.
Power Armor would be alright, except you would ALWAYS have to be in it lest be in fear of people taking your Power Armor. That would make sleeping and other "activities" very frustrating.
... maybe I'm over-analyzing.
Can I write in a vote for Nano-Augmentation from Deus Ex? I see no downside to that.