can you tell me please whats the best Windows Explorer replacement?
the fastest (very important) and most customizable
I want to fully integrate it (replace explorer.exe, replace keyboard shortcuts etc) can I do it with a Windows Explorer replacement or I need a shell?
First of all, I don't think there is one "best" Explorer replacement. Everyone has different needs, and while some prefer Directory Opus, others prefer Total Commander or any other file manager.
As for "full integration" you need a shell. It is possible to use a file manager, AFAIK, but it requires some extra tools and utilities (systray and hotkeys, for instance), while a decent shell does all the important shell-work.
I think the topic has been discussed before, so try a search and you'll probably find some good explanations