I can't really pick just one episode that is head and shoulders above the rest. So I'll have to go with a select some good ones.
Hmm... well, for seasons 1-4 that I've just rewatched (can't remember the later episodes well enough to take them into account yet), I'd have to go with (in chronological order):
1) Twisted. Kinda liked it. Perhaps not all that strong an episode in some ways, but watching people get lost all around the ship was funny enough to make it a good episode for me.
2) Meld/Basics. Lon Suder is an interesting, conflicted character. His eventual redemption is perhaps a bit predictable, but a good ending for the story nonetheless.
3) Remember. The story, while good on its own, is also told in an interesting way.
4) Real Life. Actually for a large part the episode is even irritating, especially the "perfect" family in the start wants to make me poke my eyes and ears out. But the ending is very strong, easily enough so to make it a good episode.
5) Worst Case Scenario. Seska's revenge from beyond the grave is nice stuff mostly.
6) Scorpion. An interesting two-parter. I only wish that Species 8472 wasn't portrayed as SO overpowering compared to the Borg. The cubes should've been more than cardboard boxes with a target icon on them. Reminds me of Star Trek Online in that regard.
7) Year of Hell. I'm usually not one for episodes that mess around with time, but overall I found this one quite enjoyable.
8) Living Witness. Now, whatever happened to the doctor's backup in his quest to return to earth... I'll never know
Probably missed a few, but I suppose this list is too long as it is
On a side note, I noticed someone liked "The Thaw". Yuck. It has to be be one of my least favorite episodes. Why is it that in so many series there seems to be an episode when there's supposed to be something weird and terrifying, it has to have something to do with stupidly colored places with retarded clowns and other such characters?
