I don’t which one is the best (because it mainly rely on which criteria) but I know the one I like the most: MD11-F .
It seems that’s it one of the favorite type of the cargo operator as well.

By the way , didn’t MDD (ooops sorry Boeing) have some project to convert the C17 into a freighter?

The MD-11 is indeed a favourite with the Freight Haulers. Fed Ex don’t seem to be able to get enough of them. It may be a failure as a pax aicraft but it makes a heck of a cargo plane.