[Micah]

Starspawn have 20, which I think it the highest base MR. There may be others as well.

Non-unique items:
Lead shield - 4
Amulet of Antimagic - 4
Rainbow Armor - 3
Armor of Souls - 5
Starshine Skullcap - 2

Uniques:
Mage Bane - 5
Amon Hotep - 5
Aseftik's armor - 3
Robe of Calius the Druid - 3
The Jade Mask - 3
Dual-wielding the Axes of Rulership will give you 2

Thin that's all of em. There are also a few spells that will give you +4 in combat, but they don't stack AFAIK. Iron Will, Resist Magic, Army of Lead, and Antimagic will all work (earth and astral).