Guardian droid
From Wookieepedia, the Star Wars wiki.
A guardian droid was a type of battle droid with armor strong enough to be considered impervious to laser fire. They were presumably taken out of use because their damage resistance made them difficult to control. The model was considered to be old by the early years of the Galactic Empire.
C-3PO once wore the armor of one of these droids, making him immune to laser hits as he uncharacteristically smashed a group of hover guards.
