My oshimen is Harunyan! I was so glad when they made her center in Heart Ereki! I can't decide on what to call her though, I usually just call her Haruna. I know the members call her Nyannyan, and that her nickname is Kojiharu. If I met her I'd probably call her Kojima-san.
And I agree with some here, you should really just have one oshimen. I like several girls, but Haruna is always gonna be #1 in my own sousenkyo.