Here is a list of most important SAP transactions while working with ABAP:

/h - switch on the debugging mode

SU53 - check user authorizations

SE11 - ABAP Dictionary

SE37 - ABAP Function builder

SE38 - ABAP Editor

SE41 - Menü Painter

SE51 - Screen Painter

SE80 - Object Navigator

SE81 - Application hierarchy

SE93 - Display / Create transaction code

SE95 - Modification Browser

SE09 - View Development Class