Oh, tough one. If it was spelled Susannah, that would be the one, but without that last h I'm just not as interested. Marianna is my second-first choice.
I prefer Susanna because it has a flowery meaning. Also, I prefer the sound of the letter S than the dure one of the letter R. Also, in Italian, "Marianna" could even be an exclamation used when someone wants to express a negative feeling.