I would go with Chryssalid, but since everyone is mentioning it, i'll suggest the Tentaculat!!! This unit is like chryssalid (zombifies) and it gave me a hard time when i was playing X-COM TFTD. The thing that made it even more of a difficult opponent was the fact that it had a lot of time units, and during its turn it could move and move and move, and even though you felt you had a safe distance, many times it would reach you and kill you over and over. Great times great games (all the x-com and ufo games)