Night Lord's Castle: Awesome. It's pretty much everything a creepy castle run by a vampire and witch should have: bats adorning the entrance and tower, a prison cell in the tall tower for the good guys, and the eerie crystal ball predicting the doom of anyone who dares oppose them make the atmosphere wonderfully appropriate. Basil's throne is a nice touch in the main room of the castle, and the large oak doors on the side give it a sinister majesty (note, I have it in the configuration of the front of the instructions/box, for those curious). Also, the fact that it's swarming with guards add to the "do not mess with us because we're evil and magical" vibe.
Alien Avenger: The epitome of a UFO. It's a giant (for LEGO) flying saucer manned (?) by three aliens and an android guy with two extraterrestrial buggies to explore various worlds. Oh, and Alpha Dragonis' ship detaches from the main vessel. Pretty darn cool. The rotating laser canons on the front and the magnetic buggy-lifting hose are great touches and makes me want to reenact a scene of aliens abducting cattle (I need to obtain some LEGO moos!).
Sorry for the lack of pictures. I promise when I get a decent picture-taking apparatus (the iPhone 3G's camera is surprisingly lacking compared to my old phone) I will post pics.
EDIT: Picture of the Alien Avenger:
I should also review some of my other 90s sets some other time while I'm at it. I miss those days.