I have been looking around for really nice drop downs that are also responsive, and I have to say, this PEN by Andor Nagy really impressed me. It only uses HTML and CSS to create a fully responsive drop down, with three levels. We can download it here, or use the version embedded on Codepen.
See the Pen Pure CSS Responsive Dropdown Menu by Andor Nagy (@andornagy) on CodePen.
I also thought this css-only dropdown was well done.