For me TOS had many great episodes in the first season, many of them already mentioned. But the one that effected me the most is THE DOOMSDAY MACHINE. As someone else said I never get tired of watching it.

Of TNG one of my favorites is Q WHO. When Q introduces Picard and company to the Borg for the first time. I think that was when the Borg were at their creepiest. Another episode was YESTERDAY'S ENTERPRISE, when the Enterprise C accidentally slips through time during a battle and disrupts history. The final battle is really cool.

DS9 has a few gems: TRIALS AND TRIBBLE-ATIONS, IMPROBABLE CAUSE, to name a few.

I didn't care much for Voyager or Enterprise.