Meterpreter

Meterpreter (meta-interpreter), is an advanced payload included in the Metasploit Framework. In my opinion this is the best payload ever. It can upload/download victim's files, read stored password hashes, keylogging, and even run a vnc server on the victim's machine. 

Meterpreter compatible with most of the exploit and auxiliary on metasploit. Making it the most sophisticated and the most used payload by pentesters and hackers. The way that it accomplishes this is by allowing exploit developers to write their own extensions in the form of shared object (DLL) files that can be uploaded and injected into a running process on a target computer after exploitation has occurred. 

Meterpreter and all of the extensions that running are executed entirely from memory and never touch the disk. That means, allowing them to run undetectably under the scan of a standard Antivirus.

Meterpreter console :

"the quieter you become, the more you are able to hear.."