Wow. That's a big question.
I can start with some of the old ones.
"Demon With a Glass Hand" and "Soldier" from The Outer Limits (both written by Harlan Ellison.)
"It's a Good Life" (written by Rod Serling, from the story by Jerome Bixby) and "Nick of Time" (written by Richard Matheson) are my favorite episodes of The Twilight Zone. Maybe also "Walking Distance" (Rod Serling.)
"The Dead Man" (written by Douglas Heyes, from the story by Fritz Leiber) may be my favorite episode of Night Gallery. It's the scariest, anyway.