Amazingly, HAL's death scene. After all his murders, his helpless pleading for Dave to stop pulling the plug on him will make you wish there was another way.
Now with video. "Stop, Dave. I'm afraid." and of course, the famous portion at 3:48 - "Daisy, Daisy... give me you-r... answer... do."
After the discussion of HAL's malfunction, his having killed the rest of the crew, and having heard his calm-in-a-creepy-as-hell-way voice for the whole movie, this Wham Line points out that, somewhere deep inside, he's as human as any of us and just as afraid to die.
This gets even worse when you read the novel (or read/watch 2010): HAL's psychotic behavior wasn't even his fault. He was told to conceal the existence of the monolith, which directly conflicted with his programming. This is what caused him to mistakenly report the communications relay was broken, and what ultimately resulted in him deciding to kill the crew: if there was no one to keep the secret from, he wouldn't have to keep the secret.