The most important part of an antagonist really are objectives, goals and plans to get there; it's no wonder so many have speeches about it. Even the force of utter unredeemable evil - maybe they want to destroy the universe because they see it as an abomination, so they'll get something that causes entropy, so they hire creatures in this universe to unwittingly get the work done, and have to craft a web of lies so no one finds out it's Cthullu hiding in the void out of reality. That's already a big set of things to expand on a character that you'll never root for... BUT, if you make them so thoroughly enjoyable fully knowing they won't win, you made them memorable without any goodness.
If you guys want to see a REALLY good unredeemable-but-sympathetic villain, check out Nox from Wakfu. Never excused, never given a second chance(or allowed to get one, period), but with a logic and a "noble goal" that makes you want to see how far he'll go.
I have both a sympathetic villain, who doesn't really think of themselves as such and has a chance for redemption if they notice that they're doing horrible things for very little gain before it's too late. The other villain however, was given a chance for redemption - and promptly stomped it a long, long time ago. They know no pleasure but power over others, and when something even hints at being an alternative, they supress it and treat like a job in fear it'll never be as good of a high. They've long stopped seeing people as anything but tools and also supress any involuntary affection, and excuse it as having seen lovers become enemies and enemies become lovers so many times that the lesser beings are predictable and only chase suffering. In a lie so deep, they've told themselves they're not suffering either, and for their own fault there's no solution other than stopping them and their plans forever.
And then there's Mortimer back in the day, a much more traditional heel face turn development. He started out with good intentions, got drunk with how easy things were(and his ego inflated that as he started to treat it like a game of wits) and eventually started causing trouble. But when Spinel and Beryl came into the scene, he remembered how helping others made him feel good and his purpose in life didn't have to be through thievery and destruction, and started changing his mind slowly but surely. It still took an extremely traumatic event for him to really drop any pretense he was enjoying villainy anymore, but give or take some questioning in a position he couldn't ignore, he would've stopped all on his own eventually.