Is it? From where? (Not doubting you, just want details).
Hamlet/Horatio would be my pick. Better if the can get all brokeback in iambic pentameter
I’ve never been into Kirk/Spock myself, since Shatner looks too much like my dad, but I have to honor the original K/S ladies for encouraging and popularizing this line of fanfiction back in the pre-internet days, and for giving us the “/”.
Frodo/Sam is my pick, since that’s what I write these days.
I also have to mention my other favorites, Garak/Bashir, and O’Brien/Bashir.
There is a serious shortage of good Lucy/Ethel slash, or Laverne/Shirley.
Godzilla/ Gamera 
The intriguing point is, while a sexual component is present only in slashfic, it is canon that they loved each other deeply – in a Platonic sort of way, according to Tolkien. (While Tolkien was intentionally writing an epic that did not focus on romantic matters, he was far from prudish in his depiction of human nature. The key to the tragedy of Turin was that he fell in love with and married his full sister. While such eroticism as existed in the major character hobbits is found in Sam and his feelings for Rosie, one wonders what exactly bachelor Frodo’s sex drive might have been.)
On the Jeeves/Wooster note, the one of the finest pieces of cross-over fiction I’ve read was a cross-over between Jeeves & Wooster and the Lord Peter Wimsey universe. It totally subverted J&W canon by positing that the reason that Wooster buzzes around like a young fathead and never seems aware of the serious world events going on around him is on account of shell-shock from the War. While I think Wooster is a perfectly lovely character in canon, just as he is, this story handles it all with exquisite sensibility and gets both sets of characters down to a T. I don’t even remember if it was slash or not, just that it was That Good.
Of course, I’m a bit of an angst junkie.
I read that one - one of the best in the fandom.
I don’t remember the episode number, but in S5, Spike says “Angel and I never! Well, except for that one…” (night? month? year? decade?) And then he was interrupted. In the commentary, Joss confirmed that the two of them absolutely had sex.
The linked picture is SFW but the site itself is full of pop culture slash art with numerous risqué images that may not be SFW. Thankfully, Star Trek is not included.
I came in here to mention O’Brien/Bashir. I watched a ton of DS9 over the summer, for the first time since I was a kid (at which time I was pretty exclusively a TNG fan and didn’t pay much attention to DS9), and I absolutely love them.
For a while around season 4, I became pretty convinced that not only were Julian and Miles sleeping together, but Keiko knew about it and didn’t mind. Then O’Brien started having all this weird sexual tension with Kira when she was carrying Keiko’s baby, and Keiko seemed fine with that too. And to further complicate things, Alexander Siddig was the father of Nana Visitor’s real-life baby.
I tend to like the darker ones. In the Harry Potterverse, that’d be Snape/Lupin (oddly I find this more believable than Sirius/Remus, at least post-Prank), and to a much lesser degree Harry/Draco, although I’m not really into the lead characters as much. And I’ll second Garak/Bashir from Deep Space Nine – that one writes itself, IMO!
Edited to add: Oh! Forgot Picard/Q. I mean, come on.
My pet theory on the Garak/Bashir/O’Brien triangle is that Julian was involved with Garak following that juicy pick-up scene in “Past Prologue,” and then switched to Miles at the end of the third season. I blame Garak’s torturing Odo in “The Die Is Cast” for this–obviously Julian couldn’t take it.
The transition between the two relationships is abrupt. In “Improbable Cause,” Julian and Garak are exchanging gift chocolates; two episodes later, there’s Miles and Julian drunk off their asses, singing “Jerusalem,” and Miles just about telling Julian he loves him.
I got a lot of fanfic mileage out of that. 
Link?
This was the first slash story I ever read, and it’s still one of my favorites.
Doctor Who/Jack Harkness is another great pairing, as is Jack/Ianto. Does it still count as slash if the characters’ homosexual relationship is part of the show?
Yes! I also re-watched a lot of TNG this summer, and I was actually surprised that my eight-year-old self never picked up on this one, because it’s so blatant.
I’m put off by Aubrey/Maturin, but I did rather like some Richard Sharpe/Duke of Wellington that I read once.
For some reason this made me picture Percy_and_Baldrick thus ruining the whole thread for me. . .
[retch]
Maybe not Percy/Baldrick, but it wouldn’t surprise me if Edmund and Lord Melchett had some hatesex going on. And of course, Flashheart seemed attracted to Baldrick, especially his beard – gives him something to hang on to.