I'm a big fan of "The City on the Edge of Forever." Harlan Ellison has a real talent for evocative titles that plant an image or an idea in your head, and I'm very glad that it was never changed in any of the rewrites.
A good title should intrigue you and give you a hint about the story you're about to see. "Mirror, Mirror" is a very good title. "The Menagerie" is very evocative. "Far Beyond the Stars" is pretty good. So are titles like "Tomorrow Is Yesterday", "The Big Goodbye", "The Trouble with Tribbles" or "Looking for Par'mach in All the Wrong Places." Those are titles that stick with you.
I'm definitely not a fan of all the generic, one-word titles that came into favor during the TNG/DS9/VOY era. It made all the episodes seem more interchangeable and less memorable. I'm constantly mixing up TOS' "Metamorphosis" and "The Changeling" just because of their generic titles. Either one of those titles would've been right at home on TNG.