First, don't just automatically assume a CSS framework is necessary, or even desirable. Depending on what you're doing and how good you are at markup + CSS, it may not be worth it. The Wikipedia article barely scratches the surface of the debate I've seen online, e.g., this criticism of the YUI Grids framework.
If you've decided that a CSS framework is for you, then you need to decide what problems it will be solving for you. Once you have that list, you can start comparing different ones to see how well they address your problems.
Finally, there are any number of minor factors that may shift the balance for your particular application. Already using YUI on the site? Then the Reset, Base, Fonts, and Grids combo may get a bump, etc.
The two I've seen reference most often are YUI's and BluePrintCSS (here's a comparison).
UPDATE: One thing I haven't seen explicitly addressed by a CSS framework is Setting Type on the Web to a Baseline Grid (see this article on composing to a vertical rhythm for more details).
