Cathexis

This episode is one of the detective stories than Voyager did regularly, and pretty well too. On returning from a shuttle mission Chakotay is found to be brain dead. Added to that, multiple members of the crew start taking critical actions of which they have no subsequent memory. It becomes clear to the crew that there is a non-corporeal entity aboard the ship that might be hostile and is taking control of the crew to accomplish what it wants to do.

The episode is filled with plenty of tension and a nice twist at the end. The Doctor was always a great character and the writers use the fact that he is a hologram (and immune to the non-corporeal being) as an interesting plot point in this episode.