your answer made me smile. it's like wishing infinite wishes to a genie that'll grant one wish :)
well if we go in the realm of impossibilities my answer would be: I'll buy a Tardis
I would buy: world peace, freedom, liberty for all, free energy, free food, free accommodation, colonize Mars, free education, free love, free chocolate, free knowledge and free will.
Free energy for all ought to be enough to eliminate the need for money. I'd buy up all the finite mineral wealth and raw materials of the world first though and dispense with it benevolently :)
Making me pick one? Technology for infinite free energy :)
I'd buy Cuba... I have a political idea I want to try out, somewhere between a Republic and all out communism... figure the people living there with Castro wouldn't mind too much more than they do already...