David at War on Guns has some great food for thought- what would you use to hunt a T. Rex with? Fairly good timing on the topic considering the Ray Bradbury based movie Sound of Thunder is due out soon. David includes some links to articles discussing the issue. I've already left a comment- a double rifle in .600 Nitro Express (or .585 Nyati) with A-Square Monolithic solids.
That's assuming I can handle the recoil on such a beast- there's no point in hunting with a gun that you can't shoot. Outside of the big bore "African" rifles I'd say that I'd most like to be packing a Marlin 45-70 loaded up with Garrett cartridges. With a squad of Marines packing FN Mags at my back.
Please do head on over there and leave your own ideas.