Just a reminder that, as some have noted, top notch bar box eight ball is alive and kicking at the BCAPL extravaganza in May, and event that, thanks to Mark Griffin, seems to get better eer year.
I think the most interesting challenge match out there would be Shane vs Appleton.
However, it needs to be noted that although both have played bar box eight ball extensively, Appleton's specialty is English eight ball, a game in which the pockets are cut like snooker pockets (not only in tightness but also in pocket shape), and, just as in snooker, certain shots are virtually impossible.
I don't doubt for a second that Appleton would also play exceptionally well on an American barbox, but he'd likely be a slight underdog with Shane, and Shane might be a slight underdog on an English barbox.
Perhaps a home and away match in which one match is played in England on an English bar box and a second match is played in America on an American bar box would work.