@Ng Wei Sheng Interesting question! In my opinion, the leader's heart is (or at least should be) serving others first, before yourself. It might be a something of a cliché, but it holds true nevertheless.

More importantly, a great leader should always want to work together with others—truly collaborate—in order to accomplish whatever the company/team etc sets out to achieve. So I'd also say that a strong sense of togetherness and compassion is the heart of a great leader.