I'm sorry, but goddamn Howling Banshees. Incredibly fast, have power weapons that tear Marines apart, and they are fast enough to hit your devastators before you can suppress them. THEN, they scream, suppressing your Devastators. if you retreat them, they take double melee damage and die. If you try to walk them out, they die.
You want to counter them with Assault Marines? Fuck you, they die. Tacs? They die. Devastators? They die. Scouts, die. Force Commander, Die. The only thing that can feasibly take down banshees is a dreadnought or Librarian, or mobbing them with everything. Then the Eldar player drops a plasma grenade on your horde and you die.
FUCK YOU BANSHEES.