Unfortunately, this is a very subjective questions. You will have people that believe that the most important theme is fate and that the most important theme is ambition. Therefore, the only answer that I can give you is one that I, personally, see as correct.
For me, I believe that the more important theme in Macbeth is that of fate. Especially in today's society, too many people place ambition on a pedestal and, in doing so, they fail to see the consequences of their actions when climbing the ladder to the top. While the consequences of this climb are highlighted in Macbeth, it is far more important to follow the theme of fate.
Fate, for those that believe in it, seems to be much more intriguing in the text. It is the supposed control that Macbeth believes that he has over his own life, and the potential crown, that leads him to become murderous. Like many things in life, people typically only listen to part of what they are being told. They do not pay attention to the whole. If Macbeth would have listened to everything that was happening around him, he would have been overly aware that fate has a place in everything.