I used Directory Opus back on the Amiga, and Norton Commander on the PC until it was surpassed by
Dos Navigatorw, which I used all the way up to Windows 98.
I use Directory Opus, Total Commander and XYplorer today, though mostly TC.
DO was an awesome replacement for the windows style folder browsing on Windows XP I think, it is an impressive program that can do so many things. It's just awfully expensive, and since I upgraded to Windows 7 I haven't really missed it enough to warrant the upgrade price.
TC looks horrible. What won me over about it was the way it allowed my fingers to work the same way they did back in the NC/DN days. If you are using the mouse then it is probably not as good as the others, but if NC keyboard shortcuts are hardwired into your brain, you feel right at home

.
XY is great. There are so many things to like, and the developer is very active and keeps on adding nifty little features. Unfortunately he keeps on adding nifty little features and not the big nasty stuff that actually makes it unusable to me

. The sad part is some of the problems are due to the language it was written in, and thus unlikely to be fixed.
I've tried xplorer2, cubic, free commander, altap salamaner, but none of them really struck that magic mix of features and efficiency for me. I liked xplorer2 a lot, and would probably have bought a license if it wasn't for the absurd licensing hoops to put it on an USB drive back then.