Without having any extraordinary attachment to Tara myself, I can honestly say that "Seeing Red" is quite possibly the darkest and most painful episode of the series (although "The Body" is up their too). I'm not saying that its
bad overall (though not nearly as well-done as "The Body", and I have
very mixed feelings about how Spike's attempted rape of Buffy was presented), but that episode is as dark as this series gets.
Although I very much like how season six wrapped up, and think that that would have made a fine end to the series. Also features arguably the most awesome moment for Giles of the entire franchise.
Angel goes darker on occasion, I think, but its not as well-done. Indeed, a big part of why I prefer Buffy is that while Buffy was often dark, and tragic, and horrifying, it also had a lot of humour and joy and shear
fun, which I often felt Angel lacked. And the darkness and cynicism of Angel often felt gratuitous and forced, to me (which is why Fred's death had almost no emotional effect on me, I think- they were trying too hard to make it a horrible tearjerker).
Although Darla's death at Drusilla's hands is one of the most effective horror scenes I've ever seen on television.
Edit: A couple more notes:
Regarding the seeming meandering of season four, not knowing what to do next, I think that's kind of the point. Its about the characters having graduated from the familiar trappings of high school, and having to try to shape new lives for themselves. I don't think its a failing of the series, or at least not entirely, but a deliberate theme.
And regarding fanfiction...
I tend to write crossover fiction, myself, and while I've never written it, I can't help but think that Darla's death on "Angel", and specifically Lindsey's role in it, basically make Lindsey the perfect crossover foil/antagonist to... Harry Potter.
Harry Potter's character is defined perhaps most of all by two traits: A very self-sacrificing love, and, ultimately, acceptance of death. This is perhaps shown most clearly in his mother's sacrifice to protect him as a baby, which his own actions in the Forbidden Forest in
Deathly Hallows echo, and in his willingness to give up the power of the Resurrection Stone.
Lindsey, in contrast, tries to "save" the woman he is in love with (or more accurately, obsessed with- I refuse to consider
that love) by having her turned into a vampire- having her metaphorically raped and then killed so that a demon can prance around in her body. He destroys her, partly out of refusal to accept her death, and partly to spite Angel.
I was toying with the idea for a "Buffy" crossover with
Harry Potter a while back, and at some point, thinking about that scene, realized that Lindsey would pretty much have to be Harry's opposite number, because his actions their are almost a mirror to what Harry would do, and believe.
Plus, wizard cop (Auror) vs. demonic defence lawyer.

It just kind of fits, doesn't it?