Bootstrap is very popular from what I read.
At first I thought I liked it, but learning just a little bit of css alternatives changed that for me.
I personally use flexbox, which I like a lot, for responsive design. I feel like I have more control, and I feel like its more predictable when I’m trying to do something fancy. That might just be because of my low skill level in bootstrap of course.
Learning more about bootstrap’s more advanced features, is definitely on my to do list though.