The Vampire Ancient (sometimes styled Ancient Vampire) is possibly the most dangerous foe in the game. Unlike Ancient Liches and Daedra Lords, they appear very frequently at higher levels and have no ranged spells—therefore, they can't blast themselves into Oblivion with their own attacks.
Ranged weapons are almost useless, since Vampire Ancients are very fast and can be within melee range before you have reloaded your weapon. Spell Absorption is little help, unless you have a lot of empty Magicka which can be restored by the Ancient's Shock spells, and these attacks are quite powerful. Characters with a capacity of at most 50 Magicka will probably not be able to absorb even a single Shock spell before it causes damage.
On the other hand, if you have a powerful Spell Reflection spell, the Vampire Ancient will kill itself easily with its own Shock attacks. And although they are immune to paralysis, they can be paralyzed if their own spells are reflected back at them (making for an easy target).