If you could only pick one episode to show a non-brony...

[Ice Fox 1,145]

Return of Harmony or Sleepless in Ponyville, maybe.

[User07832407840 177]

The first episode of Season 1 might be a good choice considering that it introduces the characters really well and ends on a really good cliffhanger that might make them want to watch the next episode to see what happens. And then they'll have watched two episodes. 

Although, Return of Harmony part one might achieve those same things PLUS having Discord's awesome character and voice, so on second thought, I might choose that....
I would also consider Lesson Zero or Sonic Rainboom. It really depends on the person.

[RivkaBobble 110]

Daring Don't.

I'd show it because it's awesome considering a pony pegasus ranger is battling someone while an over reacting fan messes it up.

It also shows mlp isn't always the magic of friendship.

[RarityFan01 2,444]

To turn them into a brony: Lesson Zero

 

To torture them:

 

Well, if it's my very uncreative family who doesn't appreciate art: Suited For Success. (I personally love that episode.)

 

If it's a random person: The pilot Episode of G3. 

 

If it's only G4: Call of the Cutie

[1Bit 1,531]

I'd go for 'The Last Roundup'

 

I just think it's really well written, the humour is wonderful and its definitely a more mature episode I guess you could say