So Pre-timeskip Law wants another throne, not One piece. I'm assuming that means he wanted to be a Yonkou. He was planning this for a while so I'm also willing to bet its not Blackbeard.. Plus it would ruin Oda's characterization of Luffy if it were. Fun loving Luffy would turn into homicidal Luffy.
I think its Shanks. Luffy wants to prove himself to Shanks, what better way than taking him out.