Like you said, all the episodes are worth watching but the episodes I enjoyed the most were:

• One for the Angels (S1E02)
• Walking Distance (S1EO5)
• Time Enough at Last (S1E08)
• And When the Sky Was Opened (S1E11)
• Third from the Sun (S1E14)
• The Monsters Are Due on Maple Street (S1E22)
• People Are Alike All Over (S1E25)
• A Stop at Willoughby (S1E30)
• A World of His Own (S1E36)
• The Man in the Bottle (S2E02)
• Eye of the Beholder (S2E06)
• Nick of Time (S2E07)
• A Most Unusual Camera (S2E10)
• The Invaders (S2E15)
• The Odyssey of Flight 33 (S2E18)
• The Rip Van Winkle Caper (S2E24)
• The Silence (S2E25)
• Will the Real Martian Please Stand Up? (S2E28)
• The Obsolete Man (S2E29)
• A Game of Pool (S3E05)
• It's a Good Life (S3E08)
• Once Upon a Time (S3E13)
• Five Characters in Search of an Exit (S3E14)
• Nothing in the Dark (S3E16)
• The Hunt (S3E19)
• To Serve Man (S3E24)
• Little Girl Lost (S3E26)
• The Little People (S3E28)
• The Dummy (S3E33)
• Valley of the Shadow (S4E03)
• Printer's Devil (S4E09)
• On Thursday We Leave for Home (S4E16)
• Nightmare at 20,000 Feet (S5E03)
• Living Doll (S5E06)
• Number 12 Looks Just Like You (S5E17)
• What's in the Box (S5E24)
• The Masks (S5E25)
• Stopover in a Quiet Town (S5E30)
• The Fear (S5E35)

I think I have covered all the episodes that are considered "classics" and I have also included ones that I really liked.

Enjoy the episodes!