Most defensive items are situational, so it's hard to list a single best build. You don't want a Banshee's veil if the enemy damage consists of a group of Trynda/Darius/Cait for example, and you won't use Thornmail much if they have something like Karthus mid, Rumble top and Fiddle jungle.
Amumu can go a lot of things. Add Aegies/Runic Bulwark, Sunfire Cape, Shurelya's, Randuins and Locket to your list of items that he should get. I would recommend removing Banshee's Veil, Warmogs and maybe Abyssal Scepter.