I would take...the Black Star. I would take it because if I possessed an such an item, i could collect as many souls as I please, and use them to possibly create more magic items! Then I would use said magic items to set up a theocracy built around yours truly and dominate the world with my magical firepower! Then I could walk into a bakery and take as many cupcakes as I want, and no one could stop me! NO ONE COULD STOP ME! Or a sweetroll. That's my 2nd choice. They look pretty tasty.
I would plant the hist tree, get some sap, buy a pet lizard and feed it to him until he turns into an argonian, give him the werewolf blood and call him Sotek, just for giggles.
I seriously love this answer in just so many ways. Although the real Sotek may feel differently.
Kind of, I wouldn't attempt to rule Brazil. Not yet, I still have to learn a lot more... And I don't need to overthrow the government, the government itself has already taken care of that.
I would, however, cause complete and utter chaos. Sadly, odds are that things would end up better than now