The Old Traditional is the best single purpose strop. Not having tried the Kanayamas, I obviously cannot comment on their quality, but I would be much surprised if a Japanese craftsman procuded something sub par.
The SRD Premium is a fantastic multi purpose strop. Great craftsmanship, too, and I don't think the Premium IV should be worse :)
But the JimboRoo simply cannot be beat, for obvious reasons ;)
Regards,
Robin