Lu Wu
Akechi Seiko
Toyotomi Kumiko
Tokugawa Nana
Uesugi Asako
Liu Sheng Er

Those are the stronger SR's in my opinion.

Honorable mention for the qiau sisters, although they are not the strongest, but if you get one early, they can be powerful. I'm still using one of them because she stuns her enemy. But I'll probably be replacing her with Eusugi Asako when I get her leveled up. Or an SSR