Chaos Space Marines: Try and rush it with Maulerfiends? Blastmaster spam? Fuck, I don't know. Sorry guys. Black Mace Daemon Prince will fuck up a Wraithknight royally but you're gonna have to land, not die, and still be able to catch it afterwards. Still, a lot of sass points if you actually do it.
Blastmasters only able to get up to one hit per turn makes that a dicey proposition (for a whole lotof points in support of fielding that one blasmaster). I've had to pull it a few times, and it takes most of my army's shooting (4-6 blastmasters, Forgefiend, maybe 2 Havoc squads) for several turns to get the Wraithknight down to a few wounds before the Wraithknight charges in and I get a lucky 6 to wound with my Sorcerer's Force Axe to ID it. That's how it's played out multiple times, honestly. And now the ID is only doing D3 wounds, so... the rest of the army shooting for a few turns to get that unreliable result is still necessary.
Alternatively I guess you could take 15 Havocs with 12 lascannons/missile launchers, alongside 15 Chosen with meltas/plasma guns, and just let that rip. You opponent will be killing those squads as fast as they can, but...
Rad Grenades are gonna be pretty central to the plan here since they're a really nice debuff (double points if you get an Allied Enfeeble to make it T6).
Speaking of Rad Grenades, I imagine an Inquisitor could pull the same. Plus, say, a Skitarii Vanguard squad in combat, supported by some sort of thunder hammer or power fist.
Mostly just having fun thinking about stacking T debuffs, but... I suppose it does point out to me that Skitarii are a thing, now, too. Ballistarii with lascannons, etc. Radium jezzails dealing 2 AP2 wounds on 6s to wound. Etc.
Last time I took one out I mobed it with Cultists and used two lords with a Powerfist to smash it. And as its not a Charitor it could not target the Lords within the 40+ Cultists that I had around it.
I think the same plan will have to be used going forward.
One things: Wraithknights have stomp now. So not only will they clear your cultists several times faster, if they roll a 6 while the template is on top of your Lord--poof!
"Hope that your opponent doesn't roll a 6 on Stomp or the D-table" is such a shitty survivability mechanic.
Chaos Daemons: Daemonettes are the name of the game here, but most of the Greater Daemons can at least chip a few wounds off. Plaguebearers only wound on 6s, but that's better than not wounding at all. Use Malefic, summon up a bunch of Daemonettes, profit. Tarpit with Grimoire-augmented units and hope for bad Stomp rolls.
One thing I want to try more is pairing Fiends with lower-I units: charging in, say, 2 Soul Grinders and some fiends for a lot of S10 attacks at I3 before the I1 Knight gets to strike. With only WS3, the Soul Grinders are hardly reliable against a fresh 'Knight, but... still might gets some wounds through. Agreed, though: Daemonettes and Seekers are the way to go, far and away. A Lord of Change with Staff of Change is a nice amount of S8 attacks--and any MC who rolled the "Fleshbane & Armorbane" Greater Reward.
Khorne Daemonkin: Bloodthirsters are probably all you've got, although gunning for lucky 6s on Bloodletters is a reasonable plan if you can reach melee. If Skulltaker still does his Brutal Legend decapitation thing, worth a shot.
Daemons having Fearless, not Daemonic Instability, helps tarpit it, Stomps aside. Flesh Hounds scouting up can get in its grill right off the bat (can still wound on 5s if you assault). Bike squads are less than a hundred points for 2 meltas... and you can bring a lot of those bike squads. Still, no good answers here. Though every one of your small MSU units that a Wraithknight destroys, they do give you a Blood Tithe point, as some small silver lining...