Riyad-us-Saliheen 845
Abdullah bin 'Amr bin Al-'as (May Allah be pleased with them) reported: A man asked the Messenger of Allah (PBUH): "Which act in Islam is the best?" He (PBUH) replied, "To give food, and to greet everyone, whether you know or you do not."
[Al-Bukhari and Muslim]
Feeding poor and destitute is an act of goodness, and so is fulfilling the needs of the indigent. Greeting everybody (saying 'As-Salamu 'Alaikum'), whether an acquaintance or a stranger, is a good manner too. Both of these acts generate mutual love and remove hatred and ill will from hearts. All other forms of greetings do not no substitute for Islamic greeting.