Well, honestly, I have never found a great argument either for or against the existence of free will. You'll hate the basis on which I believe in it because it is so terribly tautological.

I believe I have free will because it appears to me that I have free will.

Because I can't make a solid logical argument either way, I believe the thing that brings the greatest amount of joy to me. I never forget that there's an asterisk on this belief, though, and am ready to abandon it should an argument come along that removes ambiguity.

But I also believe that "belief is the death of intelligence", and so use "believe" as shorthand for "have as a working hypothesis".