Depending on what you are doing, you might choose different Frameworks.
Foundation & Bootstrap are both great - if you want all that overhead. I personally don't like the grid bootstrap comes with that much, but if you are building an extenisve application and are likely to need all those UI elements these are great choices. The more Bootstrap elements you need, the better option it becomes.
For building simpler sites, or where you want to start from more of a blank canvas, get the simplest thing possible. Depending on your targeted browsers, how much you want to keep layout classes out of your mark up you could choose from:
http://cssgrid.net/ -super simple. Almost not a framework.
http://susy.oddbird.net/ - focuses on allowing you to define your own grid & allowing you to keep layout classes out of the markup. Sass based
Note on preprocessors - If you are getting into mobile and using complicated frameworks like bootstrap or Foundation your css is complex. Strongly recommend using a preprocessor to help with that. That might feedback into your choice of Framework - Bootstrap is based on Less, Foundation on Sass