1. Heldrake with Baleflamer. I can stay in my Rhinos, in which case I can't shoot properly and my Devastators die, or I can disembark, in which case it Vector Strikes, killing 2 or 3, and the Flames the remnants, killing a huge number. As an AV12 Flyer, it's very difficult to put down.
Flakk missiles, and plenty of them. You can spam them quite easily thanks to Tactical Squads and scouts. This option is the deciding factor for me whether to get Missile Launchers or Plasma Cannons.
2. 20 Cultists. Autoguns and a Heavy Stubber, sat in a piece of 4+ cover. I cannot apply enough firepower or get close enough to charge. They shrug off the scant firepower I send their way and 9 times out of 10, capture an objective.
Ravenwing Land Speeders with Heavy Flamer, Attack Squadrons with flamers, Land Raider Redeemer. Essentially anything that ignores cover.
3. CSM with Mark of Slaanesh and Icon of Excess. They almost ignore boltguns, and 1/3 of Plasma shots you send their way come off the invulnerable (added to 1/3 missing, that's a large decrease in firepower). In combat, they strike before Tacticals and with more punch.
Deathwing. A simple squad with Power Fists or Thunder Hammer will send them packing. And you don't have to worry about initiative.
4. 3 Nurgle Obliterators in seperate squads. Very difficult to get rid of before they kill a Terminator squad or expensive vehicle.
Use DarkShroud(s) to cover your advance, while fast-moving Black Knights or plasma-toting Attack Squadrons isolate and eliminate them.
5. Daemon Prince with Sigil of Corruption, Wings, and Mark of Tzeentch. Flying with a 3+ Invulnerable re-rolling 1s is pretty damn tough. Today it charged a squad of Deathwing, before being charged by another squad. It ate both squads at the cost of 3 wounds.
Do they have Eternal Warrior? If not, a Librarian with Power Mace will work a treat. If so, then kite it at 24" with Ravenwing Bolters.
Bit of a rock-paper-scissors scenario here, so you'd probably have to select 2 or 3 of these at a time, as you probably won't be able to fit in all of them.