OK, I'll bite. Mainly because some of my faves aren't getting enough votes.
Include:
Barovia & Strahd (hardly needs to be said, does it?)
Darkon & Azalin
Nidala & Elena (Nidala needs more detail, yes, but not to be thrown away)
Invidia & Gabby (She's the ultimate "home-grown" darklord. Daughter of a vistani and Vlad Drakov (Implied), killed her mother with the (unbeknownst) assistance of Alfred Timothy. Victim of the Gentleman Caller, mother of the Dukkar, what's not to love? And how can anyone read or play The Evil Eye and not fall in love with Invidia?)
Falkovnia & Vlad (I know a lot of people think it's time for Vlad to die and have someone else takeover, but I've got a soft spot for him.)
Harkon Lukas & Kartakass
Lamordia (I'm OK with giving it to the Dr., though. I buy Mangrum's explanation that Lamordia reflects the Dr. and not Adam as part of Adam's curse to be unaccepted, even by his own land. But I don't know if I buy the "born evil" path to darklordhood.)
Dementlieu (Love the place. Like Dominic, but I'm not too attached to him. Maybe off him and give the place to The Brain.)
Mordent (Sorely in need of fleshing out. Godefroy is a fine DL, but I'd be interested to see another spin on it.)
The Shadow Rift (respin it how you like, call it the land of Faerie, hide it away in a nearby plane, whatever. I happen to like the big gaping hole in the world, but that's just me. Wouldn't mind a more appropriate DL, though. Not Loht, though. He's more interesting if mobile, IMHO)
Malken (Might be time for Tristen to go and Malken to move onto another host, but I like him, as Romir's ghost, rather than a hereditary curse.)
Ivan/Ivana (does that count as two or one?)
The Nightmare Court
Ebonbane (Yes, in direct opposition to my dislike of "born evil" darklords. But I like what his presence means for the whole Shadowlands cluster. And with all the detail he's gotten since his first appearance, he's no longer the "silly sword darklord" to me.)
(On the lack of any non-canon domains: No offense to any who've written them, but I've rarely seen any that really grab me.)
Exclude:
The Three Hags (I like Tepest well enough, but I never see any real hooks to use with the sisters)
Avonleigh (I think the Shadowlands cluster needs more civilization and less abandoned wasteland)
Sithicus & Inza (should have just let it dissolve when Soth left.)
Valachan & Von Kharkov (they've tried and tried to make it interesting and it just doesn't work for me)
Tristessa & Keening (just seems like leftovers to me)
I'Cath and Tsien Chiang (Better a
real Chinese-influenced domain or none at all. I much prefer domains where someone can actually be from, and things can happen that don't involve the DL or their supporting cast.)
"We're realistic heroes. We're not here to save the world, just nudge the world into a better place."