This one is tough for me since I am so partial to villains with shades of gray. They're just more interesting. So, my favorite "pure evil" villains tend to be on the more demonic side.
Terry Brooks has a number I'd call out. "the demon" from Running with the Demon and Findo Gask, from the same series (he survives the initial series and shows up later). There's other great examples: the Reaper from Elfstones of Shannara (can't wait to see that on screen!) and the Morgawr from the Voyage of Jerle Shannara. Needless to say, I like straight up evil, as long as it is demonic.
If you extend it to videogames, I *hated* (in a good way) Loghain from Dragon Age: Origins. No way I was sparing that manipulative SOB, even if his daughter pled for it, which is why he worked well as an antagonist (he also had some gray in him, which made me like him more - he saw himself as justified). And of course breaking my "shades of gray" rule, Kefka from FF VI. Holy hell, was he awesome(ly evil). Dude pretty much destroyed the world for giggles.