Mine's the Body Shop black liquid eyeliner. It's the one item I've worn every single day for years, I think since the 90s or early noughties at least! I love it because it's a felt pen-type of nib so easy to draw a line with (can do it in seconds) and it when it dries it stays BLACK, not turns grey or anything like others do.
I know there are others to experiment with, but if it ain't broke etc...
what's yours?