Bootstrap was built by Twitter and is one of the most if not the most popular of them all. It probably has the most features of them all, allowing you to build everything with it – fixed, fluid or grid-based is easy to make with this framework as well as making layouts mobile ready.
This is a great choice as Foundation gives you a great, flexible grid that can be nested with ease. It’s focused on mobile, thus allowing you to build mobile first and add stuff for other versions later on so you website will really be mobile optimized – useful tool for rapid prototyping so it’s not meant just for designers.
A wise choice – it has it all to get you started – flexibility, customization options, 960px wide grid, uses SASS ( adding amazing features to CSS3 – nested rules, cariables, selector inheritance and much much more). Gumby also helps you make clutter-free responsive site.
Skeleton is not just your average framework, but more of a development kit as it will help you with quick web development with much needed tools such grid, buttons, forms, custom typography and more.
Unsemantic is built with Compass ( CSS authoring framework – gives you cleanr markup without presentation classes, sprites in a splash, mixins, …) and SASS ( see explanation at Gumby ) so works really well in responsive type of websites using Adapt.js as well ( Adapt.js is a lightweight JS file that determines which CSS file to load before the browser renders a page). Cross-broswer compatible and SEO optimized.
So this is it – the choice is yours to make as it all comes down to preferences and features, but whatever you will choose with any of these frameworks you just can’t go wrong.