To me the pinnacle of icons was always the MagicWB [0] / MUI [1] style, even though I never really had MagicWB on my systems. Maybe strange choice of colors (pale blue, pink, orange?), but nice dithering and color interplay to create neat 3D illusion built nicely upon the OS 2.0 look.

[0] https://en.wikipedia.org/wiki/MagicWB

[1] http://www.sasg.com/mui/preview.gif