I will be off topic as what I am going to suggest is not open source... But I myself looked for a long time what was the best free windows file manager and I settled up on FreeCommander.
By default UI is not nice but it's possible to make it nice enough.
I use this one mainly because : you can use it for free, dual pan, it remembers tab history between sessions, tabs can be renamed and colored.
Hope someone will suggest a good open source one but I don't think there is any :-/
Double Commander is free and open source. I've been using it for a long time. I'm not sure which one I used before, but could very well have been FreeCommander.