I always preferred v4 over it's later v5 incarnations. It's incredibly powerful straight out of the box, but if you take some time and configure it to your taste it becomes a dream of a file manger. I simply couldn't live without it on my Amiga.
Development carried on with Directory Opus well after it was a commercial product after the author released the source code. It's a testament to the high regard the product had that it was being developed until 2004. The last commercial release was v4.12, but over at
http://dopus.free.fr/ you can get v4.17pre22 for free which is much better and gives you things like glow icon support that came after it's last commercial release. Don't be fooled by the low version increase, there is an obvious reason why they couldn't advance it to version 5 which it deserves.