I have found Word to do a good job. I make sure that every bit of the document has been assigned a style - never use Ctrl-B for bold or anything like that.
Then save as a filtered web page.
In Sigil, I delete all the style lines that Word puts in the head element (usually about 500 lines!), then enter the link to the external stylesheet where I have all the styles listed.
I then do a search and replace for all the MsoNormal styles and replace them with a regular paragraph.
You now have a clean document!