I've played against Dark Angels [Termie Heavy] with a single knight a couple of times, and I've played against White Scars with 1/2 Knights a few times now.
I've been trying to figure out Blood Angels while doing so. I've also been mixing some IG bits in, as well, and also played straight Guard once.
My experience is that you need to tailor your list towards them. You can't make an "all comers" list at 1850 points that can handle 2 Knights when you don't have one. The very best thing to kill a Knight, is another Knight.
At 75 points more than a pair of LRBT's, you have the same effective shooting power, the same number of hull points. You have comparable armour, shooting wise.
You also get...
1: A decent Invul save against shooting.
2: Double the movement.
3: Effective immunity to the damage table. A MEQ with Lascannon has a 3.7% chance to inflict an extra d3 HP, cut in half if on the Ion Shield side.
4: Full fire power down to the last HP. When 1/2 Russes die, you lose half your shots, not even counting shaken / stunned / Weapon Destroyed effects.
5: Effective immunity to assault. I mean, compared to a Russ? 3 more points of Armour, a WS, plus you're going to pound the bloody stool out of the guy charging you / being charged.
The only "weakness" of single facing shield requires a "sacrifice" unit that is going to knowingly do dick all to the target... and it's going to be your strongest shooting, let's not kid ourselves.
So looking at it from the BA side of things, I'd need to drop 3 Melta-Assault Squads with TRIPLE Melta [Total of 360 points] around a single Knight to reasonably try to off it in one turn. By reasonable, I mean that with 9 shots, I should hit 6 times, inflict 5 damage results [pre shield!] and hope to get an explodes or two from those hits... and pray that the shield doesn't make that effort all for naught.
I'm committing all FA choices to this single effort, and 3 pods, meaning I must take at least 2 other pods to ensure I can do this first turn... with no backup for them.
I don't have great odds of success, and failure to kill that one knight will then result in...
Knight moves, double taps BC onto one squad, probably wiping them. "Split fires" the stubbers at squad 2, so he can assault them. Charges them, probably wipes them [5 man squad, no way to fight back really] and maybe even works it to stomp into the 3rd unit.
So having commited the very best of my AT to the cause, if I don't take down one Knight first turn I have no real way to continue killing the 2nd Knight, as I can't catch it. It just runs away from my Melta, blasting them with a 2x BC.
[Gotta run back to work, I'll try to finish this tonight. Knights are brutally difficult to deal with from a TAC perspective.]
Carrying on...
A Marine with a Powerfist, that gets 3 Attacks, would have a 58% chance to do no damage to a Knight, a [very close to] 34% chance to inflict a single HP, and then an 8% chance to cause more than one HP to it... at Initiative 1. Why did he charge a Knight? I dunno.
So the Knight presumably charges them with a powerful HoW hit, then attacks landing an average of 2 hits that typically wound... and then stomps on the unit. It's reasonable to expect a 5 man MEQ unit to be reduced to a single survivor in a typical round. Lulz, then you're tied up in CC during your opponent's shooting phase, huzzah!
The Knight creates an effective "don't go here" zone 24" accross, if not more. It can move a foot one direction, or the other, and then charge. I don't know Daemon Princes that well, but they need St 8 at a minimum to be any kind of real threat to a Knight. At S8, WS 6, that Prince has a 22% chance per attack to cause harm. With 8 attacks, that Prince still has a 13% chance to do 0 HP.
Without factoring "Explodes" results into the mix, a Prince with 8 Attacks has the following odds of damage:
0 HP = 13%
1 HP = 31%
2 HP = 31%
3+HP = 25%
For what it's worth, with 8 Attacks the odds of NOT inflicting an Explodes result at all are 86%, so you usually wont inflict an explodes result, even with the equivalent of 8 PF attacks at high WS.
The best thing to tarpit them with is tactical marines, of all things, simply owing to most stomps only punching through Carapace armour. Anybody taking 10 man units of Tacticals these days? That wants to try to tarpit a Knight... that's shot them 2 or 4 times with a Battle Cannon, for a couple of turns? Nobody?
So you can take them with Assault Terminators... eventually. Many turns to do so, and probably at the cost of a 6-8 man unit of them.
Laughably, because they're so tall, it's easy for them to march into a ruin mid-field, claim cover from all directions, and still have a great field of fire, because the end of the cannon will poke out of the cover they're in. They don't have to worry about a firing point like a Dreadnaught might. So you could run [probably outside of cover for a turn or two] at a Knight, get shot 2 or 4 times with a Battle Cannon, to try to oust it from a Ruin. Of course, if you DIDN'T kill it while within 12" of it, you'll be double-tapped and then charged [HoW, D-Class Attacks, Stomps]. Vehicles are absolutely annihilated in CC against Knights, as 4/6 stomps inflict an auto-pen, while 1/6 destroys you outright. That of course assumes you survived the D-Class attacks at initiative, as a vehicle that's not a sure thing by any means.
Board control like a... I don't even know. Face off against a pair of them sometime. It's not fun.
With a pair of them, you have incredible board control. If you spend half your points on Knights between 1500 and 2000 points, you've got a list that will be tough to beat.
[Need to pick up kids from the sitter's, I'll come back to this tonight. I'm still not done.
]
Enter the most powerful, seemingly unbeatable list I've ever played against. No really, it is. I've never played against a list that I actually asked my opponent to not play again, except this one. In 20+ years of playing GW games, I've never said, "I won't play against that again without warning. I'll just quit and we can play something else." I did that against this list.
1850 pts
750 - 2x Knight Paladin
[White Scar Marines]
225 - Chapter Master: Bike, Power Fist, Shield Eternal
125 - Storm Talon: TL AC, Skyhammer Missiles
125 - Storm Talon: TL AC, Skyhammer Missiles
184 - Bike Squad [4+1]: Meltabombs, Combi-Melta, 2x Grav Guns, Multi-Melta [Attack Bike]
184 - Bike Squad [4+1]: Meltabombs, Combi-Melta, 2x Grav Guns, Multi-Melta [Attack Bike]
184 - Bike Squad [4+1]: Meltabombs, Combi-Melta, 2x Grav Guns, Multi-Melta [Attack Bike]
[73 more points spent somehow... maybe CC upgrades, I think?]
That's the jist of it, anyhow. Chapter Master joins a squad. Knights lead the charge, with the bikes just behind. Highlights of the list...
Everything Jinks, at a MEQ level, except the Knights and they just get an Invul instead.
List is effectively immune to terrain, as bikes have skilled rider, Talons fly over top, and Knights move "quickly" through terrain anyhow, not to mention they love getting a cover save.
Knights are weak against flyers. Storm Talons are some of the most points efficient anti-flyer in the game.
Everything is extra tough. Bikes, Knights, or hard to hit Flyers. Oh, and then they Jink.
If something moves up beside the Knights, the bikes can move to intercept with Grav, Melta, AND assault. If they're stuck in cc, they can Hit and Run away 2/3 times, so that the Knight can Double-Tap, the Bikes can shoot, and then everything assaults again! FUN!
By working in waves, if a Bike squad needs to Jink, they can retreat and another unit can quickly redeploy to take over the shooting... or they can just jink and still assault with bonus strength HoW attacks.
So while surrounding the Knights seems like a good idea, you then place yourself in range to be shot up by bikes, then assaulted by "everything" that then breaks off and you can then be shot again. Not that much survives being in close proximity to 4 battle cannon shots, 18 Grav shots, 3-6 melta shots... and then being assaulted by everything.
FREE Bonus! Hiding something out of LOS? The Chapter Master can drop Max Strength, Melta-AP Pie plate on you, even while moving because he has relentless and it's Barrage! JOY!
So Knights are powerful, and if they take up 1/2 ish of your points you can then take support units to offset their VERY limited weaknesses.
Terminators are good against them, except for the Melta-Version that I haven't faced. Grav just laughs hysterically at them, though.
Flyers are good, but if you take counter-flyers you're set. Even a pair or trio of Hunters / Hydras are good.
Nothing wins in CC against them, except maybe Assault Termies, and they're slow enough that the Knight can just walk away from them, killing whatever it needs to on the way.
They're the "perfect" unit, and easily worth 500 points each, in my opinion. They're full throttle until the last HP is gone, and that takes a lot of effort that tends towards the suicidal end of use... so you don't have anything left to take on the 2nd one. I very much do not enjoy playing against them. I'm going to proxy 3 in the next game I play against my friend with the above list, to try to prove a point about how "unfun" they are to play against. That's my rant.