▲这个 of 表示比较范围，意为“在……当中”，相当于 among。又如：

Of all the choices, this is the best. 在所有选择中，这种最好。

Of the two coats, the black one is the better. 在这两件外套中，这件黑色的更好些。

▲你的句子意思是：Of Syria’s neighbors, Lebanon has been the most deeply impacted by the influx of refugees fleeing the violence, with over 1.1 million registered refugees from Syria.  在叙利亚的邻国中，黎巴嫩所受因躲避暴力而涌入的难民潮的影响最深，已登记有110万来自叙利亚的难民。