I think it's obvious - the best single shot rifle cartridge is the one that you have in the hands.

The project that led me here is a single shot pistol, a Thompson Center Contender. The fellow who I work with had handed me a 10" hybrid chambered .410/45 Colt barrel.
This is one of the earlier ones without the rib. I'd been thinking about using some 444 Marlin brass to span the long corridor of throat to get a bullet closer to the rifling leade. The Marlin brass is too short, and I ended up blowing out some 9.3x74r brass to a roughly 45 x 2-7/8" case. I've had to turn the case mouth for .006" neck walls.

The last load fired in there was 61 grains of 3031 with a 300 grain XTP

Brisk, with room for more ass.
Haven't affected the brass at all at this point.
It's ridiculous - absolutely silly, but it's a single shot and it's a hoot and a half.