While I have serious doubts this is the most sophisticated trojan/back door/spam bot program extant, there is no doubt of it's advanced features and capabilities. I just think the most sophisticated program of this sort is yet to be discovered, simply because it is so advanced. It's running quietly on some thousands of machines, doing it's worst, without being detected.
Despite that qualification, this Tech World article outlines a truly advanced piece of malware. It attempts to eliminate competitor's malware on the machine as some earlier malware has, but does it in a significantly more capable and reliable way; by using a corrupted copy of Kaspersky Anti-Virus. It also uses encrypted instruction sets so competitors cannot co-opt the program for themselves and can be updated to take instructions from another server,should the author's server be down or taken away. All told, it is pretty stout stuff. Check out the article, which is easy to read and understand, plus having the desired attribute of being brief.