That's a VERY tough call.
OK, I am going through a list and pulling out the ones I remember as being great:
*All of the Jack Klugman episodes. He was just the perfect Twilight Zone guy, including In Praise of Pip, A Passage For Trumpet, and A Game of Pool
*The old west one with the penicillin
*The Invaders, just a classic
*The hour-long Robert Duvall ep., Miniature
*To Serve Man (how can you not love this one?)
*The old Mexico one with the peacable space man who gets gunned down, The Gift
*The To Kill A Mockingbird one, with the two kids, The Bewitching Pool
*the Elizabeth Montgomery and Charles Bronson Adam & Eve one, Two
*One For the Angels, i get chills just thinking about it
*The Robert Redford as death episode, Nothing in the Dark
*Shatner with the fortune teller machine (this one is so unvelievably awesome); also Nightmare at 20,000 Feet. Shatner was a Twilight Zone god!
*The Lonely, with Jack Warden alone on a planet with a robot; what a shocking ending for the '50s
*The Four of Us are Dying, another ep. that gives me chills
*The Big Tall Wish (chills)
*The Howling Man (scared me when I was a kid)
*Billy Mumy and his grannie, Long Distance Call
*The Mirror, with Peter Falk
*The Grave, with Lee Marvin
*Once Upon a Time, with Buster Keaton
*Five Characters in Search of an Exit, with the five people trapped together in a circular room
*The Fugitive, with the old man from space. Love that one...
*Little Girl Lost - with the portal in the bedroom - CHILLING!
*The Monsters have Landed on Maple Street
*The Dummy, with Cliff Robertson
*Living Doll, "I'm Talking Tina and I'm going to KILL YOU"
*Uncle Simon, with the robot who is just like the lady's uncle
*Number Twelve Looks Just Like You, one of my all-time faves
*Night Call, with Gladys Cooper as the old lady getting calls from the cemetary. Spooky.
*Ocurrence at Owl Creek Bridge, what a cool one.
*Nan Adams on the highway, The Hitch-Hiker, "I believe you're ... going my way?"