I'm inclined to say "private office", but I've never had one, so it might not be all that great. I guess it depends how open is open. At my job, there are 6 people in a "team room" that is partially open to the hallway. Communication within the team is improved, but it seems that anyone coming to interrupt anyone else also interrupts me. For a lot of stuff it doesn't really matter. For some things it is really hard to make any headway unless I know I'm not going to be interrupted for several hours.
I think the best design would strongly encourage people to use shared workspaces but have private offices for everyone that wanted one.
Large space with several co-workers. That's ideal for me. If I'm ever 'in the zone', I just put on headphones even if music isn't playing.
If you had your choice, would you want your own private office or would you prefer working in a larger space with several coworkers?