.32 S&W Long
I think I would like to have a "First Series" Colt Detective's Special in .32 Colt New Police, aka .32 S&W Long.
Missouri Bullet Company has stopped producing these for now, but I am hoping they will resume production. Their "Cowboy #18" bullet is a .313" diameter 78 grain round nose bullet with a BHN of 12, stock no. 313078CB. For authenticity, I wouldn't want the hi-tek coated bullets.
Mo. Bullet Co. "Cowboy #18":