My favorite TOS episode is and always will be THE DOOMSDAY MACHINE, with a great guest performance by William Windom, an awesome original score by Sol Kaplan, and one of the best "countdown" endings ever. Least favorite: THE ALTERNATIVE FACTOR, which, to me, is just a total mess.

Favorite TNG episode: Jack B. Sowards' WHERE SILENCE HAS LEASE, the coolest and eeriest episode ever, with an intense
"countdown" ending that's even better than the one in THE DOOMSDAY MACHINE.

COMPUTER VOICE: Sixty seconds to auto-destruct.
DATA: Captain, we are clear of the void. You may stop the
auto-destruct.
PICARD: Data, warp six--any heading!
RIKER: Captain--
PICARD: It could still be part...of the illusion...Data, report.
DATA: All navigational systems confirm--we are at warp six-- on course...
COMPUTER VOICE: Forty seconds to auto-destruct.
DATA: ...to the Morgana Quadrant.
PICARD: Counsellor...
TROI: It's gone, Captain. I no longer sense its presence.
COMPUTER VOICE: Twenty seconds to auto-destruct.

No one even breathes for a long moment...

COMPUTER VOICE: Ten seconds to auto-destruct.
RIKER: Captain...
PICARD: Abort--auto-destruct sequence.
COMPUTER VOICE: Riker, William T. Do you concur?
RIKER: Yes! Absolutely! I do indeed concur wholeheartedly!
PICARD: A simple "yes" would have been sufficient, Number One.
RIKER: I didn't want there to be any chance of a misunderstanding.

2nd favorite TNG: the one with Data's daughter. That one actually makes me cry.

Least favorite TNG: any episode in which that stupid holodeck is allowed to threaten the ship or the lives of its crew, or is used as a crutch just to allow the writers to make up dumb detective stories and Worf westerns and stuff.

Favorite Voyager episode: YEAR OF HELL

(Also agree with everything Sparkie just said.)

Legend, oh legend, the third wheel legend...always in the way.