I'd say maybe a Lich. But then I use MMM that add more powerful Liches (eg Lich King, Lich Lord, Deep Lich), so I'm not sure about vanilla. Liches don't move fast but they attack very hard, and have spell reflection which can be deadly for a mage.
I vote for Liches too. MMM adds some really insanely strong Liches, that I have trouble killing with even my highest level character, who can take out just about anything with one arrow.
And they usually just happen to spawn in swarms, so you'll have three Lich Kings, two Lich Lords and a few normal Nether Liches all attacking at once.
Vanilla Liches are pretty bad ass too, but high-leveled Grummites, Flesh Atronachs, Scalons and the second Gatekeeper probably take the cake over them.