There are no best server as none of them host any files, just catalogue them and sources. Due to source exchange you will eventually get sources from across the whole of the ed2k network. They are not eMule servers by the way they are ed2k servers. Basically the "safe" ones are RB 2 and 2.1 (any other RB server is a fake), eDonkey servers 1 through 6.
I would say the Bytedevils servers too, but unless you are using a mod which shows country flags I wouldn't recommend as there are now a number of fake BD servers out there. If you are using a mod that allows the flags then the fake BD servers are the ones with the US flag the ones with the German flag are the legit BD servers.