I'd rate the quality of the sandbox elements of a game based primarily on how much there is to do besides kill things. GTA:San Andreas and Saint's Row 2 are the biggest,best sandbox games available...on the consoles. Most anything else is more a blood bath than sandbox(again,on the consoles).
No, I take that back. The Sims: Castaways takes the cake! There is even actual(virtual) sand on the island!