I am quite impressed, especially with this particular episode, The Doomsday Machine. This episode was somewhat influential on my young life (according to Wikipedia, I would have been either five or six years old when I first saw the episode on TV). How was it influential? Well, it scared the crap out of me! :) I had been watching Star Trek most weeks with my Dad during the original broadcast back in the 60s; however, after watching The Doomsday Machine, I was too scared to continue watching the show. By the time I had screwed up my courage to start watching again, there were only a few more episodes left to air, the series having been canceled.
Two points of interest about this episode:
This first video is when Commodore Decker (played by William Windom) is in command of the Enterprise with Captain Kirk aboard the damaged Constellation:
This next video is of the very end of the show:
And this final video is a compilation of various visual effects from the enhanced episode. There are some nice touches in this compilation you should look out for; for example, at the 1:24 mark, an asteroid strikes the hull of the Constellation and shatters into several smaller fragments. I thought that was pretty cool.