B'Elanna Torres seemed to go just about as far as she could thanks to a strong early push sooner than some of the other characters. "Barge of the Dead" is the point where the writers start to figure out there's more that can be done.
Much of the character's previous experiences were incredibly insular. Sure, early on she developed a weird bond with Harry Kim, had the support of Chakotay, and dueled with Joe Carey before earning his respect, but it wasn't until she started a romance with Tom Paris that B'Elanna started to play nice with others in any real sense. Even most of her spotlight episodes kept her isolated, in sometimes imaginative and tragic ways. Her struggles with the Klingon and human parts of both her biology and experiences were confined in that inner world, too.
Again, "Barge of the Dead" changes all that. Here we meet her mother (took six seasons!), thanks to an elaborate setup that drops her off in the Klingon afterlife, which is itself significant as one of the more intimate looks at an alien culture in Star Trek history (showing rather than telling, which is what many, many people who believe they know what storytelling should do are always, um, saying).
For the rest of the series, everything we thought we'd concluded about B'Elanna becomes fodder for further expansive exploration of the show's most dependable dramatic subject. That makes this one essential on every level.
Not bad for a season that didn't seem to know how to do anything right. Well, take that as a suggestion about the real worth of the year.
franchise * series * essential * character
Memory Alpha summary.