Eduardo,
ABAP, because it helps you to understand how to use tools as VOFM, BADI, userexits and other enhancements. VOFM is usual in pricing, account determination, output messages, copy rules and so on.
What are the ways a SD consultant can start learning the above?
Also, how (ways) can an SD consultant learn ABAP? (that much ABAP that could help him/her in his/her work)
Thank you, in advance!