I wouldn't even mind the whole dash priority if Riot could just give us a list like "these are S tier CC dashes, these are A tier, B tier, etc..." and "higher tier CC dashes will always get priority over lower tier ones, while CC dashes in the same tier will give priority to whichever is used first.
Alas, they can't even do this. These are spaghetti coded and the result is random, more often than not. Or counter-intuitive.
If I use J4's E-Q, it actually works the way I want it to if my Q connects to my E AFTER any other CC has hit me. If my Q connects first, any displacement on me will deny my dash AND deny the knockup. I can use E-Q to go through a Lee-Sin, and he can buffer-input his R on me, and he will literally kick me out of my E-Q and I won't knock him up, even though I not only casted first but was mid-dash through him before he used his ability. Meanwhile if lee kicks me, and I drop E-Q during the animation with the Q reaching the E after I start getting knocked back, I'll pull myself through him afterwards.
Now, I can say let's ignore J4 since his E-Q is a "special case". Maybe he's just the problem because the dash comes after the abilities are cast, it isn't the actual ability.
So a Hecarim is charging at a Poppy. He's clicked on to her so his E should knock her back, and then she presses E to charge him. Who knocks back who? Well, I guarantee you it's possible that Poppy knocks back Hecarim because that's happened to me, and right after it happened there was a forum post asking why everything got priority over poppy's dash cc and I said Hecarim's E didn't and the OP said it was Hecarim's E that got priority over his Poppy charge. Err... opposite experiences? It's purely random. You can't possibly say a Hecarim that's mid-e and targeted on Poppy without losing vision "didn't press it fast enough". Nor can you say that a Poppy who has started her animation doesn't have the knockback. It comes down to whether the game chooses to let Hecarim start his knock-back animation early enough to knock-back Poppy first, or too late that he gets knocked back first. Which probably comes down to how far away Poppy was when she managed to press her knockback (so it's not actually random, but it appears that way).
I think the only fair solution is also quite simple:
Any CC from an enemy champion gets priority over the champion's own forced CC unless otherwise stated.
So in the J4 case, it states he moves to his flag despite CC and is an exception.
In the case of like Malphite, the ultimate is unstoppable so CC obviously doesn't interrupt.
In the Poppy v Hecarim case, BOTH champions get knocked back and CC'd (with neither champion appearing to have been hit by anything after the initial collision). The impact knocks them both back, as if both lost priority. It's the only fair way, unless Riot wants to make an explicit priority list.
and 
can still flip him off
's silence prematurely ends ww ult as well, didn't even know about the voli interaction :( feels bad man, ultimate rendered useless by basic abilities.
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