Witch King is insanely powerful thanks to his crown, he's always a good choice. My opinion doesn't change regarding the others, I posted this knowing about their extra cost to use each ability.
Khamul's rules have stayed the same since his release, any topic about him across this site is still valid today. The only difference is that he and the Witch King are the only two Ringwraiths who get Heroic Strike. He's still a very good choice, his magic is harder to cast but you can always throw more dice because he'll regain his will, he can be FV6 to beat most Elves and match most big heroes, he's great.
Dark Marshall: He has a natural FV6, which is great, and you probably don't need to use the banner effect every turn. The first few turns of combat, sure, then pick and choose depending on how much of an advantage you do or don't have. He's still awesome.
Shadow Lord: If your opponent is shooting you, you're within 24" of them. Your worst case scenario is that they call Heroic March and back up 4.5" a turn while you advance 6", then you're only gaining 1.5" a turn and the Shadow Lord will run out of will protecting your models. That scenario is kind of impossible because most boards aren't long enough for it to happen. More likely is that you close the distance in 3-8 turns, depending on Heroic Marches and if the opponent backs up. You're not losing too much Will protecting the army with that, and he's still got all the benefits of being a Ringwraith on Fellbeast once you're close enough.
Undying: You have to spend Will to regain Will with him, so that ability is only useful if you have multiple spellcasters. What I consider his main ability is using his Will points as Fate, that's still there, he's still very hard to kill.
Betrayer: If you're taking him in a Mordor army, you're not using his Master of Poisons rule anyway. You're using him as a flying Spider Queen with magic. Think about how powerful the Spider Queen already is, now lose one point of FV and the Broodlings to add courage, flying, durability, and spells. That's a scary thought.
This edition's nerf was needed because they were all so powerful before. They're still really powerful and game-changing, just require a bit more practice.
So my answer is the same as it was in my first reply: There's no "best" Ringwraith, there's no Ringwraith that will always be better than another, it all depends on what you want your army to have. Undying is a great leader because he's so hard to kill. Khamul is a beast in combat. Witch King, if you have the points, will dominate the game. Tainted, if you have Shamans and tough troops, will ruin your opponent's plans. Betrayer can assassinate most heroes in one turn. Dark Marshall gives you a banner,
I've recently learned that banners are pretty much essential to any troop type Mordor has. Shadow Lord can still protect your army from shooting long enough for you to get into combat. Knight of Umbar still has 3 Might, doesn't lose Will when he win his fight, and Combat Mimicry was situational anyway, it's not something you use every turn. Dwimmerlaik had the biggest nerf, but there's still a 6" bubble of fear around him and you can use that.