Strongest CC unit? Normally, I wouldn't hesitate to say "Seer Council", but since you excluded HQs, I'll have to come up with something different.
Let's see... Wyches are resilient vs. Power weapons and strong vs. dual wielders, but will falter against swarms. Strinking Scorpions are good against any non-elite non-powerweapon units, but suck against the aforementioned. Banshees excel against heavy-armor-reliant troops, but have a hard time vs. swarms and Wyches. Genestealers and Possessed are good against almost any kind of units, but are expensive for what they do, and the former isn't very resilient, either. Monstrous creatures, like Wraithlords and Carnifexes, as well as CC vehicles like Dreadnoughts, will eat through swarms and lesser infantry, but are vulnerable to shooting.
What I'm trying to say is: It depends on the situation.