If you're starting from a completely blank slate and you want a canned look and feel that saves you the trouble of having to concoct one yourself, it's hard to go wrong with Twitter's Bootstrap. HTML5 Boilerplate is best if you're constructing something totally anew or you want to pour an existing design into a new framework for easier manipulation. 52framework is most useful for its HTML5-specific extensions, and both 320 and Up and the Less Framework are handy if you're building something with a tight focus on small-screen devices.
A quick note on the scoring below: Features refers to the overall bundle of capabilities supported directly by the framework. Setup reflects how easy and intuitive it was to get the files, unpack the whole thing, and deploy it on a server. Configuration reflects on the ability to make changes, including the use of online tools that let you create a deployment bundle. Finally, Extras refers to anything useful that's not part of the core framework, such as Bootstrap's plug-in architecture, Boilerplate's integrations with frameworks like Rails and Node.js, and 320 and Up's Font Awesome and test scripts.
Twitter Bootstrap If you want to start with both a Web framework and a design -- a clean, appealing design at that -- Bootstrap is one of the best places to begin.
Originally developed by Twitter as a framework for its own GUI, Bootstrap has been released as an open source project under the Apache License 2.0. Its success, and value, is reflected in the sheer number of sites that have made use of it (the developers maintain a gallery of "Bootstrapped" sites), as well as the galaxy of third-party themes and templates built with and on top of it.