1. Murray (slice bh counts, puts him above Djokovic by a hair)
4. Del Potro
Indeed. People seem to undermine variety in conversations like these, but Murray's is ultimately why I gave him the (*slightest*) edge as well. Consistently, Djokovic probably gives you the heavier, more forceful ball from that side. But Murray can willfully hammer the ball and change directions with his backhand as well. The real difference then, as I see it, is how varietous he is with spins and tempo.
At their best, I might favor Djokovic's ironclad stability and added weight, but Murray, to me, can be more effective on average. They're so close that it's a very nuanced discussion, though.
On a related note, Del Po's lack of variety is why I ranked him lower than most of his peers. He, too, can consistently give you a very forceful shot, but he's also very predictable in that sense. Rarely does he ever send it down the line, and when he does, it can be very inconsistent. That's one thing he did a lot better in 2009, in my opinion.