The mind transferrer from Star Trek.
Just imagine what you could do if you could put your mind in somebody else's body, over and over again.
As good as being immortal, plus all the variations you could experience.
Second choice would be the replicator from Larry Niven's Ringworld.
Far better than the Star Trek ones, it absorbs molecules from the air, it's portable, contains its own power supplies, and can make pretty much everything from food and medicines to planks of wood.
Try getting an 8ft length of 4"x2" out of the Star Trek ones!