Personally, if I could only choose one, I would have the portal gun. Imagine the possibilities. The world would be utterly chaotic if everyone had one, but if it was only me then what fun it would be. I guess you'd need the long-fall boots too, could be ugly otherwise
Of course if you owned the patent to a working lightsaber then you'd make a millionty billion pounds as literally everyone aged between 8 and 50 would buy one. There would be some "challenges" faced by Accident & Emergency departments but it would be totally worth it
The gravity gun would be excellent too. Again, certain challenges faced by A&E ("what happened?" "I shot him with a toilet"