What is, in your opinion, the best definition of strategy? There are many opinions by experts such as Alfred Chandler, Michael Poter...What's yours?

In my humble opinion, a strategy is a systematic plan of action which allows its creator to utilize available resources and engage all relevant stakeholders in the most efficient manner towards the realization of predefined objectives.

of course we all agree on all the experts' opinions of how they define the strategy. 

and my opinion is not far but from my point of view; we can say Definition started with the Manager who has leadership skills to allocate Human, financial , physical , informational resources, by using the functions of Planning, organizing, Leading and controlling, and allocate these resources efficiently and effectively, to create competitive advantage in order to move forward to the organizational and social goals.

Strategy is about being different. It means deliberately choosing a different set of activities to deliver a unique mix of value; infact the essence lies in choosing to perform activities differently than rivals do !!!