TNG: "Genesis." C'mon, Barclay turns into a spider-monster. What's not to love?
TOS: "Gamesters of Triskelion." Even as a kid, I knew this one was corny and cliche, and Shawna's tin-foil bikini would embarrass Barbarella, but I still enjoy it occasionally, if I'm in the right frame of mind.
And I sometimes think I'm the only Trekkie in the world who always eagerly looked forward to "Catspaw." It's the one-and-only Star Trek Halloween Special for the Old Ones' sake--written by Robert Bloch, no less!