Yes, like Slayer-fan said, the Gorepack is the strongest formation, but the second strongest is probably Khorne's Bloodstorm (and it is the only way to get a Heldrake into a Blood Host detachment). Heldrakes are one of the best units Chaos has (even with the nerf).
As for strong units, Chaos Lords on Juggernauts are pretty badass (give them some combination of Axe of Khorne, Lightning Claw, Power Fist, or Goredrinker and watch the carnage!

). If you take a Juggerlord, you really need to take some Flesh Hounds (this is where the Gorepack comes in handy), bikers, or Spawn to help keep him alive. Maulerfiends are good apparently (I don't have one, but I've heard great things), especially if you run more than one. All three flavors of Bloodthirster are pretty good (pricey though, in terms of points and money!). Soul Grinders are not bad, and can take different weapons depending on available points (although phlegm bombardment is probably the best).