Dagger would be the most Practical.
I'd say the Portal Gun, because that'd make traveling so much fun and would be super awesome, but that thing is hazardous! Remember kids, the device has not been proven to be completely safe. Do not touch the operational end of the device. Do not look directly at the operational end of the device. Do not submerge the device in liquid, even partially. Most importantly, under no circumstances should you-- bzzzzzt~
I don't know how to parachute (I'd need the dagger a couple of times if I tried the Grapple hook/parachute combo) and the Gravity Gun would be cool for the first couple days and then be put a side as a gardening tool or to rearrange furniture.
Though the hidden assassin blade would be great. Almost no need for eating utensils. (Unless it's for soup)
Ocarina of Time would be nice... I could repair objects, change the weather, warp, call horses... Then again The endless bag of coins from Wario Land:Shake it would be nifty, it not economy ruining.
So many possibilities...
I'll have to go with Dagger... so long as I can get away with carrying a concealed blade longer then my hand around without getting weird looks.
In fact, let's combine Ezio's blade and the Dagger. It's concealed, can send you back in time, and you'll always have something to eat with.
Though I really do want that Portal Gun.