All Four of them! Double Jeopardy is not in play if a jurisdiction can lay claim to your criminal action, so if you stand on the Four Corners and shoot a man in any of the four states, each state has a right to charge murder. In addition, the Federal Government can have a go at you because you crossed state lines while in comission of a crime. Plus the Navajo Nation, which controls the reservation land the border is on. So that six separate charges.
That said, the state the dead body occupied at time of Murder would have the best case for action and the other three would likely let that state try you first. The Feds would only step in if each of the four states failed to convict, though they can step in whenever they want (they are just watching to see if you'll serve time first). Not sure at which point Tribal jurisdiction applies, but given that this is a fairly common Jurisdiction issue, I'm sure it's been worked out.