People have really strong opinions about many CSS frameworks, or using frameworks at all. One thing to note - Bootstrap is used on a good chunk of websites online, so if you have to pick a framework, it might be the most supported right now which makes it a safe choice.
Ultimately though, the best framework will be the one that's tailored to the type of site you're building, and since individual projects vary, their needs vary too. I think it might be better to have 'closer fitting' CSS framework that solves only what you need to rather than a large 'one-size-fits-all' framework that likely brings a lot of things you don't need as well.
Personally I don't use frameworks in the things I build, but I do have 'snippets' of code that get re-used in many different projects. In each case I'm trying to tailor my CSS to each site so it's a nice custom fit :D