I choose Eminem. Eminem no doubt is the biggest rapper ever. His songs are of all types, and they contain some kind of Eminem. I like his songs extremely. When you talk about 2pac, I don't know about him anything. I have heard more than 100 songs of Eminem, and trust me, they are really nice!
I think it's not right to compare them. They have different styles. 2pac died many years ago, but he is in our souls, because we're listening his songs. 2pac songs- are real rap songs. Eminem is also cool, but his songs are more soft, nearer to pop-music style. All-in all both of them are great musicians.