My favorites:
Yesterday's Enterprise -- an obvious choice, but the first time I saw it, my jaw nearly hit the floor. Not only did it
look cool as hell, but the writing was absolutely perfect.
Face of the Enemy -- a great ep for Troi, and one of the best and most intense Romulan episodes since Balance of Terror.
Data's Day -- a great view of normal, everyday life onboard the Enterprise, as seen through the eyes of Data. I just never get tired of it.
The Enemy -- a really gritty episode, with a fantastic setting in the stormy planet, a frustrated and vulnerable Geordi, and a great moral dilemma for Worf.
Q Who? -- I used to rate BOBW higher, but now I actually find myself loving this one a lot more. It's dark and creepy as hell, it's perfectly directed, and Q is at his absolute best.