Which Star Trek: TRS episodes should I watch?

After watching the new Star Trek films, I want to watch the all the best episodes of the original series. Can you recommend some?

Also, there are a few that I want to see but I don’t know the episode title…

The one with the thousands of little furry creatures.

The one where they come in contact with there evil counterparts with the goatees.

The one were Spock and Kirk are forced to fight each other to the death.