Let's not forget Yuuko in that list up there with Kaorin and Yossy
That being said: Sayumi was good at promoting the girls at other places, like on radio or TV. Unfortunately it did feel like she didn't prepare them enough for post-Sayu MM, and as soon as she left the media became more disinterested. Momoko definitely trained CG well, considering that all members are great at talking nowadays. Having seen Nanami just yesterday she seemed somewhat nervous when performing (and I just started babbling when shaking her hand so I probably confused her ) but as soon as it was her turn during the MC she was a real pro. Same with the other CG girls - I always thought of Morito as being really shy, but while she might have been the shyest in CG she's already quite good at talking in MM.
Another good point with Sayumi's leadership (to not only list bad parts) was that it seemed like she brought the group together. After she left the group became a collection of smaller groups of friends imo.
^^^^ Ikuta with the sunglasses