Murray. He is the least likely to get upset by lower rank players, so he'll gonna reach more later stages of tournaments. The only player that can beat him regularly is Fed, and Fed is getting old. Murray got time on his side. Once Fed start to fade, Murray will start winning slams and be #1.
When Delpo is fit he is the one less likely to get upset. A good attacking player can always trouble Murray in a good day, that doesn't happen as often with Del Potro.
People are talking about Murray only losing to Federer as if Federer is the one who always takes him down at the Slams, but Del Potro has played agaisnt Federer in Slams more often then Murray.