I have to agree with J.A. Hunter. In my opinion, the .500 NE double is probably the very best in balancing raw 'whacking power', penetration and shootability. If I had to buy a double again, I would probably choose this caliber and use GSC expanding Flat Nose solids in it for stopping situations.

My .470 can do pretty much the same however, when loaded to sufficient speeds with the right bullet: I've shot 500 grain TB Sledgehammer solids into media at 2100 f/s and GSC 500 grain FN solids at almost 2300 f/s into the same media. The slow Sedgehammers basically just drilled holes, as was to be expected. The fast GSC FN solid however, drilled even further while showing the disruptive qualities of a soft. That is my personal experience. If you furthermore look at field reports of .470 owners who reload (for example on reloadersnest.com), it seems that stopping power in a .470 makes a big step forward from about 2200 f/s on, regardless of bullet. So my best bet for a stopping rifle would be a hot-rodded .470 or a .500 NE. Going higher has more drawbacks then advantages in my opinion, especially in regard to recoil, weight of rifle and penetration.