TNG - "The Measure of a Man" - I was a fan of TNG from the start, but this was the first real bona fide success and from there, I just *had* to see every episode I possibly could.
BSG - "33" - As others have mentioned, this episode sold the series in a big, big way. Whether or not you like where the series ultimately ended up (I didn't like it), there's no denying the near-perfection that this episode represents.
Beast Wars - "The Trigger, Pts. 1 & 2" - This was a fabulously constructed, action-packed, mature-themed episode which hinted at some truly exemplary developments for this series.
The Wire - "The Buys" - The third episode into the series and, when it ended, I was completed taken aback. I had no idea that nearly an hour had gone by. I was so thoroughly engrossed in character, plot, theme, social commentary that ending the episode felt like cutting off the oxygen from the room.
Lost - "The Moth" - The incredible symbolism displayed in the episode about Charlie facing his heroine addiction was phenomenal. From there, I knew that the show, and the writers, knew what they were doing.