Fear is real...
The word "Love" is in the same class as the word "Intelligence"...It's something that we humans have created and have no real OBJECTIVE definition of...
But fear is fundamental to almost all mammals...
But to answer your topic...There is really no "greatest" motivator...
However, psychologists have shown that anything we associate with "negativity" is not a good motivator for most of us humans, things like anger, fear and etc
It's better to be motivated by things we consider to be positive such as love (even though it doesn't really exist, it's a powerful human concept), respect, admiration and etc...