Like Phils just wrote, it would depend on whether or not I could still handload.
If I could, it would depend on whether or not it could be custom or not. If custom, it would be a Model 629 with a Model 624 barrel installed, cut to 5" with a ramp front sight. If custom and I could NOT handload, it would be my 8-shot Model 27, cut to 5" and then hard-chromed.
If it had to be stock and I handloaded, it would be Doc 44's 5" Model 629 with the factory 5" barrel. If it had to be stock and I couldn't handload, it would be my 5" M-686 with the thinner barrel, the stocking dealer special.
Hard to do if limited to 1 Smith & Wesson handgun. I wonder what people would say if there was no manufacturer mandated? Or, not limited to a handgun?