Likely Lee Harvey Oswald, but that doesn't mean that he was acting alone. The fact that he was killed before he could testify could indicate a cover-up.
I believe that Dulles orchestrated the assassination. The CIA had been assassinating democratically elected leaders in every far corner of the globe, if they were willing to overthrow the government of Guatemala over some bananas, I find it hard to believe that they didn't have a plan for what to do in the event that a US president went against their interests.
Dulles had both the means and motive to pull it off and cover it up afterwards, that doesn't conclusively prove he did it, but it's enough to establish reasonable suspicion.