Tar Man. Oh dear God, Tar Man. The other zombies are no slouch in the nightmare fuel department either, especially the zombie in the mausoleum in II, and all of the zombies released from the Trioxin canisters in III.
The Half-Zombie's conversation with Ernie is genuinely creepy and brings up some rather disturbing possible implications. If it hurts being dead, is there the possibility that the afterlife in Return of the living Dead is actually being stuck in your corpse slowly rotting away and being a zombie just allows you the method to temporarily relieve the pain of rotting away?
Its more likely that being forcibly reanimated and trapped in a decaying body is whats causing the pain.
The idea that, no matter how much damage you inflict or what you do, the zombies will never die. Headshot? Nothing. Dismemberment? Just more pieces to come after you. Nuclear strike? Good job, now you've spread the infection even more. Extinction just seems inevitable.
Actually, electrecution and burning kills them. You just shouldnt do the last one because that just turns the virus airborne as well.
This is a truly disturbing bit of Fridge Horror to consider. The corpses sealed in the canisters were put there because the military found it impossible to kill and all they could do was confine them. So from 1968 to 1984 those corpses were trapped in small, confined, dark tanks, unable to move, unable to see, unable to hear but fully able to think and contemplate their existence, as well as feel themselves rotting away and thus being in a state of constant, unending pain and not even able to scream. Fast forward to the movie when Frank and Freddy break the canister holding the Tar Man and he's set free, but not before being subjected to the feel of his rotten flesh melting off of his bones before finally getting loose.
Sure, it's funny, but "Send. More. Paramedics." These bastards are smart enough to fake distress calls.