For iron tsuba:
Artistic talent (design, composition):
Kaneie, Nobuie, Yamakichibei
Quality of workmanship and material:
Nobuie, Yamakichibei, Kaneie, Kanayma, Owari sukashi, some ko-Shoami, some ko-tosho and some ko-katchushi
Now, this is all very subjective and many more names can be added. Also many of the "schools" are just constructs (kagamishi, ko-tosho, ko-katchushi, etc).
I think we shouldn't use the term "koto" for tsuba - it is not a period, just the name of swords produced roughly before the end of the 16th c. Pre-Edo would be a better designation.