It depends on what "rare files" you are looking for. Some networks have this files but others don't and vice versa. It also depends on the person who has the files is online or not. Emule has the largest network, so the chances of getting "rare files" are high. Some people who has "rare files" uses some other client other than emule so you have to use those clients to be able to download it. You cannot really say which client has the "rarest files". It all depends on what you are looking for and when you are looking for it. There are just too many conditions that you have to consider. Like I've looked for an old Japanese movie for 2 years through various clients and at last I found one that finishes downloading (has a complete file) that isn't on emule. Though it is the same torrent file, downloading it through different clients yield different results.