I saw you posting in the threads of @Elfyau and @Izzi and thought maybe I can help you with your question about wich covenant to play.
A basic rule of thumb: If you like any covenant ability, just stick to it. At the end it’s about having fun with your class. Every covenant ability feels different.
BUT: If you are looking for the best performing covenant for high m+ keys or CE raiding, looking at sites like subcreation can help you. They don’t show you “the best” covenant per say but what most people are playing at the highest level (high m+ and mythic raiding). And most of the time, they are right when it comes to performance.
Looking at the statistics there and reading some guides, e. g. wowhead guide on Covenants for Enhancement Shaman, you can this information together.
tl;dr : To get KSM or AOTC it does not matter what covenant OR class you play, as long as your performance isn’t too bad.
I hope this helps.