Sanwa and Seimitsu buttons are ABS as well as the knock offs. At least for the snap in ones. Not sure about screw in.
iL is probably the only ones using nylon for buttons. I really loved how solid their buttons felt. Only thing is I don’t like how they activate the switches, the lack of choice in color, and the long length making installs hard to do.
I’m mostly happy with my Seimitsu screw in buttons. RG switches also feel really nice. Not sure about Cherry MX but the mechanical keyboards I tried out weren’t feeling it for me.