Everything. I love them all. I have great memories with vampires, dragons, beholders, mind flayers, goblins, giants, skeletons, ghouls, orcs, aboleths, wolves, gnolls, demons, modrons, drow, Gods, mephits, huecuva, bears, slaadi, invisible stalkers, retrievers, etc...
But if I have to pick one: Oni. They're highly versatile. They're honorable (lawful), but truly evil and destructive. They have magic and melee, deception and destruction, flight and freeze. When players face one of my ogre mages, it is usually built up. And that is only when I do not build them up.
One of my iconic villains is a CR 25 Gargantuan Oni Lich with Archmage+ Magic, Metamagic, Masterful Melee capabilities (including a sweeping glaive attack that deals ridiculous damage, and gets to keep on attacks so long as each creature is felled by it), and abilities to move through shadows, planes and minds in ways that PCs struggle to fathom. He is my favorite villain - because he has all the tools to be the greatest villain in my world, but he is perpetually stuck doing what he needs to do to survive and can't afford to spare the time to be the menace he so desires to be. And what he needs to do to survive saves countless others in the most brutal way. It isn't the cliche 'ends justifying the means' - it is the 'ends regardless of the means'. And it is delicious to run, terrifying for players to experience, and something that just makes me smile every time I get to use him. And all he really is is a leveled up Oni.
I also have other Oni that are CR 7 variants with different weapons (bows, whips, mancatchers) and spell combos (at will - phantasmal force, misty step | 1/day - Thunderwave, Zephyr Strike, Enemies Abound, Wall of Force).
Bonus points: Mimics are hilarious fun. I love using them in ways that PCs don't expect. When the PC leaps through the air to try to grab your mimic and you ask them to roll an athletics check and they cheer making it - only for the ledge they're grabbing turns out to grow a mouth and tries to eat them.