Amumu needs as first learn tantrum for armor and some damage.

I use armor masteries, best for amumu.

Items are for tank but Crystal staf is here for some amumu damage. As amumu is using despair on enemy champion he can using this item slow enemys speed and he giving him very big damage. And dont forgot on Sunfire cape, its very good combination with amumus crystal staff and despair.
At last, amumu is cloting to tank/rescuer. On end best item archangels armor.

I am using runes on dodge change, its often better than armor. As tank amumu needs to rescue his teamnates, and with big evasion he can rescue teamnates and him.