Fallen Captains for me, especially the majors or Reaver Captains. Their shields seem to be pretty stout and when they decide to rush you it is a total kamikaze job and they don't back down. Also just seems anytime I go into POE on the Fallen map I'm a Captain magnet and 2-3 of them come at me at once, lol I would say Hobgoblins/Wire Rifle Vandal snipers in a tie for 2nd.