If I should pick only one framework for all my projects, both prototyping and production, I would go with either Bootstrap or Foundation and then improve them depending on my needs, eventually by using nice ideas from other frameworks. Since I'm a LESS user, I will probably go with Bootstrap, but if Foundation would have been build with LESS too, I would have been in deep s****.
- Bootstrap: http://twitter.github.com/bootstrap (in version 3.0)
- Foundation: http://foundation.zurb.com