One of my favorites I watched last night was Episode 4, "The Naked Time". This is the one where everyone on the Enterprise catches a virus that makes them go crazy. Even Spock, who gets all teary...over a girl! George Takei aka "Sulu" has said this is his favorite episode, probably because he's running around without a shirt and brandishing a fencing foil--you go girl! :) I'm also convinced that this has to be one of the favorites of Star Trek 2.0 director JJ Abrams as there were a number of bits that ended up in the new movie; Spock admits to being in love with a crew member (and not the same one in the movie!), Kirk and Spock get in a big emotional fisticuffs, Bones shoots up an out-of-it Kirk with an antidote, and there is a spacey/trippy time travel moment at the end. Hmmmmm.
As they say, if you're going to steal then steal from the best. And this one is a TOS classic. Next on my viewing list, "A Piece of The Action" where the crew all end up in the 1920's brandishing "heaters" instead of phasers.