Personally, I would say either Bridle Gossip or Friend In Deed. After that, show them the first two episodes just to give them a good rundown on who the main characters are, what the EoH are, and who the buck Princess Luna is.
While those are all very good episodes, they would probably give newcomers the wrong idea about the show, since two of those are adventure episodes and the other one is a very random and wacky episode.
Spoiler