Please find me codes to hack airtime mtn

In this post I will round up a collective list of MTN USSD codes (Unstructured Supplementary Service Data). It may not be a complete list and / or guide as this post title suggests, but I do want to believe I have listed most if not all the important MTN USSD codes. But, with YOUR help, we can make this list complete. So if there is any USSD command I have left out, please mention it on the comments section below.
I have also tried to provide steps (where necessary) on how to complete an intended USSD queries. I have also included links to more broader posts which have thorough information on a particular subject.