"Judgement" is one of my most rewatched episode for the same reason I like "Fallen Hero", also mentioned by HR
above. It gave us Kolos, an unconventional Klingon more preoccupied with the matters of the court than with those of war, much less belligerent than the Klingons we'd met before and much more inclined to think objectively about the state of the Empire. Of course, the character was written that way, but I think much of the credit should go to J.G. Hertzler. I loved Martok and I loved Kolos. He injected both characters with a certain air of wisdom that is very refreshing to see in a Klingon.
On the pure fun side, Archer saying, "Death to the Empire!" has got to be one of the most badass moments in the series.
There's almost a comic book feel to the whole exchange between Archer and the Klingon captain.