The letter to the Hebrews is anonymous. So, all theories of authorship are conjectures. However, this fact has not stopped scholars from trying their hands at discovering a possible author. There are three favorites: (1) Apollos, (2) Barnabas, and (3) Paul. There is some merit to each of these theories, but also problems. For instance, Apollo in Acts 18:24 is described as: Jewish, from Alexandria, educated and knowledgeable. All of these elements fit in well with what one would imagine the author of Hebrews to be, but in the end, there is no evidence that Apollos wrote it. One must bear in mind that plausibility alone does not prove anything.
Barnabas as a candidate is a bit stronger, because there is, at least, one ancient author who attribute this book to him (Tertullian), but in the end one cannot be again certain.
When it comes to Paul, there is again plausibility, but nothing more. Why did not Paul sign it as he does his other letters? Why is the language and style of Hebrews so different from what one would expect from Paul? Origen, an early church father, probably stated it best when he wrote that only God knew for sure who wrote the epistle of Hebrews.
Most likely i would say Paul wrote it...it had to be one of Paul's letters but im not too positive though