My favorite encounter... wow that's a tough one. There are a lot of great encounters. S1 ToH has some great ones. "Sabre River" has my single favorite scene that I've ever had as a player while playing a published adventure, even if I don't think it would work with me now that I'm older, it was amazing for me as a junior high kid. I3 Pyramid and I6 Ravenloft are filled with really great encounters.
The one that gets talked about the most in the current campaign was one of my own devising, the inner sanctum of the temple of Karophat, God of Technology. Or, as my players like to call him, The God of Traps.
The room was a very typical small temple with inner and outer sanctums, and like all the rooms to that point, the walls and ceilings were lined with spiked iron grills. They'd inspected these iron grills in the outer sanctum, and found them firmly attached to the walls, but the party was increasingly unnerved. One party member, the brash young Paladin walked into the room to inspect the relics and paraments on the table before the altar. I did some die rolling, and proceeded to describe the sacrificial implements, hangings, and so forth on the table in more detail - some of which were clearly valuable. A second party member, seeing that nothing ill had befallen the player, set out across the room.
What the second player did not know is that the floor was sprinkled with pressure plates, which the first player - by sheer dint of good fortune - had managed to accidentally avoid on his progress across the room. The second player was not so lucky: "*click* One of the floor tiles gives slightly, and there is a whirring sound. A harpoon flies out of the ceiling, partially penetrating your leather cuirass. Take 6 damage. Immediately, the cable upon which the harpoon is attached retracts with great force, and you are reelled up off the ground at great speed, and find yourself pinned by your chest to the ceiling. The force of the impact does a further 2 damage, and further, you find one of the iron spikes is now pressed uncomfortably into your shoulder - take 3 more damage."
So now the PC is held to the ceiling and has lost a good portion of his hit points. He calls down to the other players, "Hey, I'm stuck. Help me get down." The other players look between each other and say, "Which of us is going to go get him?" No one wants to go, so they all sit huddled in the doorway waiting for someone to take initiative. The player who plays the Paladin is like, "I'm wearing 60 pounds of metal. You know I'm on good at climbing or any of that stuff. I can't get him." So the player on the ceiling, left to his own devices, starts trying to get himself down. A strength check and a successful Heal and Jump check later..."You manage with the help of your dagger to rip the harpoon out of your flesh and armor. Take 1 damage removing the barb, and leap down nimbly to the floor." The player then says, "I'm going back to the safety of the door."
I do some more rolling: "*click* *click*", two harpoons fly out of the ceiling and simultaneously strike you in the back. Take 11 damage. There is a moment of tension as the two harpoons struggle to pull you in separate directions before the one that is buried less deeply is ripped forcefully out of your back. Take 3 damage. Now you whirl up to the ceiling where you are again impaled on the wrought iron spikes. Take 7 more damage."
Player, "Ok...so I'm unconscious and dying."
"The sailor's body is pinned to the iron grill, he's clearly badly injured at this point, and a steady stream of blood is now dripping on to the floor."
A series of skill checks later, they manage to retrieve the body mere moments before the blood loss would have been lethal. The Paladin heads back across the floor.
"Behind you a white mist begins to fill up the room."
The party fled, never to return.
