.600 nitro express comes to mind. However, on a more modern plane the .458 and the .460 are probably better. 12 gauge with slugs is always a great stopper at close range!
I always like the .505 Gibbs, for romantic reasons.
The Short Happy Life of Francis Macomber, by Hemmingway. Pity Hemmingway was a Democrat. A little repressed class warfare comes out in that one.