Barge of the Dead
I'm a sucker for B'Elanna Torres episodes, and I think this one is the best. First off, the visuals are some of the best things Voyager has done since probably Scorpion. While a little Dark, I think the scenes on the Barge captured that hellish atmosphere very well and then when we get to Grethor and you see the Klingon symbol only it's upside down, that was some great stuff. This is Klingon hell (Is the Barge of the Dead Klingon purgatory?), and the visuals showed that really well.
Setting the visuals aside, this episode is about B'Elanna, and her having to confront her anger and personal struggle. I don't remember if she changes after this episode, but this episode brought the character all the way up to the surface. We see this journey and introspection and spirituality and it's such a beautiful thing to watch. I loved the Voyager eternal hell, her vision at the beginning with the metal, and that scene at the end where she throws the Bat'leth and then hugs Janeway at the end. This episode was such a joy to watch, and while Klingon culture was getting a little stale at this stage in the franchise, this felt like a breath of fresh air. I think this might be my favorite episode of Season 6, and it's my favorite Torres episode of the series.