Total commander, for sure.
It's a replacement to windows' "file explorer",
with integrated viewers,
seemless ftp integration,
seemless archive integration (e.g. you can just drill down into arj, zip, msi, iso, as if it were yet another directory),
tons of useful plugins,
much better performance of explorer,
better exception handling (e.g. while copying lots of files of which some are dups),
tabbed interface, (all your locations are accessible and remembered for next time)
integrated command line tool,
much more customizable than other file explorers,
full and consistent keyboard shortcuts
i can't imagine using Windows without it.
i heard there's a linux equivalents (like mc for terminals, and krusader),
i used norton commmander on the DOS, then FAR, then Volkov commander)
p.s. i don't agree with "@Kwang Mark Eleven". there are many "blind spots" that aren't getting any attention by people because of lack of awareness to the fact that "things could've been better, if we just used X"
p.s. 2 i am not affiliated with total commander