If I only had one (and only ONE!) epp too watch it would be...

[Balrog]

Balance of Terror

 

[TerragonSix]

"Where No Man Has Gone Before"

(For a early episode, it had it just about right)

 

[Bones_McCoy21]

Damn, this is hard. So many choices.
I'd have to say "This Side of Paradise."
Spock's last thought, "For a moment...I was happy." That line always gets me.

my favorite quote though is "I am what I am Leila, and if there are self-made purgatories, then we all have to live in them."

I'm having a hard time deciding between For the World is Hollow and I Have Touched the Sky in which McCoy (who stole my heart)is happy. And The Paradise Syndrome in which I was truly happy for Kirk being so happy.

So after some thought I decided on The Paradise Syndrome for my favorite epp. I cry every time I watch it

Spoiler: The Paradise Syndrome

When Kirk's Wife dies with his unborn child after watching him chase her in the woods I cry so hard when he kisses her for the last time and she dies in his arms.