The first grid I used was called Skeleton. Once I got the basics for what the heck I was looking at I found it very easy to learn and work with and was a great way for me to conceptualize responsive design. It is pretty straightforward and seems to be pretty strictly percentage based with everything going to 100% column width in small viewports.
At some point later I moved on to working with the Bootstrap grid. It is more robust, as it allows for you to specify different column setups for different viewports and have nested grids. I find the syntax of using bootstrap clunky but effective (
col-sm-6, etc). Also, bonus points for Bootstrap grid in that a lot of people will be able to understand and use this grid so that is definitely something to keep in mind if many different people will be touching something over the course of the things lifetime.
It seems to me, though, that page to page designs for projects don't tend to shift monumentally enough to warrant a super complex and malleable grid system like Bootstrap. Also, while the idea of a 12 column grid laid out in a pretty Photoshop document seems like a good idea, more often then not I will get something that 'technically' fits into said grid, but because of how the background flows, or some other obscurity, it has to be custom worked anyhow.
With projects that I have been working on, I kind of like the idea of treating various page and container-type-things as their own mini ecosystem. Though, this can pretty easily breaks down right now because you have no way of really identifying how large the container is with CSS, only the viewport. If a container is right by another container things will most likely get ugly.
I'm also very much interested in seeing how CSS grid will fit into this conversation. I've seen a few talks and videos and such related to it. It's on my list of things I should play around with and understand more.