I would say definitely Dawn. She was the only one really close with all the scoobies, and her eventual research skills were actually pretty useful. After that I would actually say Oz. No he wasn't there the whole time, but when he was to me he felt the most a part of the group. He was friends with everyone, and he was pretty useful. Anya, Cordy, Tara, Angel, and Spike all have certain things keeping them back. Anya was probably the most useful out of the girlfriends, but she was never properly liked or accepted by the gang. Tara was eventually close with some members, but she still always felt like an outsider, and didn't really do much. Cordy was also never a proper part of the gang. Neither were Angel and Spike. Out of pure usefulness I would say Spike and Anya, but the whole package Dawn, then Oz.
I would also say Kennedy is very useful, but not close with the group