That's probably why I disliked crossovers so much for so long. But I've learned that they, like everything else, can be totally brilliant in the right hands. And, naturally, examples of the right hands follow.
Best FF That Proves I Was Right All Those Years Ago When I Said That Metal Control Was a Fairly Lame Superpower: Curiosities, by penknife. Harry Potter x X-Men movies, Tom Riddle/Magneto, Magneto/Xavier (sort of). This, right here, was the first story I read that made me think crossovers could be more than just a gimmick. Because, well, I never in my whole life so much as wondered what might happen if Tom Riddle met Magneto, or how a battle of HP magicians v. XM mutants might shake down, but I was gripped by this story anyway. The story works, in large part, because Penknife keeps everyone soundly in character and blends a tiny portion of the two universes without breaking either one. Read it and behold the mighty power of the Penknife, which is definitely better than metal control. (And, for the record, if you're ever offered a choice between magnetism and telepathy? Pick telepathy. If you're even tempted to go the metallic route, read this story first.)
Best FF That Proves That Anything Can Happen in the Arctic Circle: Denser Still the Snow, by Jane St. Clair, 3jane. Due South x Buffy the Vampire Slayer, Ray Kowalski/Benton Fraser/Oz. Well, now, this is a perfect example of what I mean about anything working in the right hands. I looked at the fandoms and the pairing and knew it could not work. Right then, I was ready to write an essay, complete with quotes and possibly even illustrations supporting my thesis that this could not work. And I read it, and even though there was a lengthy period in the middle where I wasn't sure if I wanted to embrace Jane or send my second to her door at dawn, I was just in awe. It works, it works, see how it works.
Best FF That Proves That Zen Is Applicable to All Walks of Life but Not Helpful in All Situations: Zen, by basingstoke. Due South x Homicide (I think), Benton Fraser/Ray Kowalski. I read the first part of this story thinking I knew who the, um, mystery detective was. I was wrong. (I Googled his name to find out what fandom he comes from, so if I'm wrong, someone please tell me.) I'd never heard of the guy before reading this, so obviously I can't say that he's in character, or that the two universes are nicely mixed (though, really, cop shows seem to go together pretty well most of the time). But I can say that this guy blends almost eerily well into the due South world. And I can also say that I read "Zen" and suddenly wanted to know much more about this intriguing character. Which demonstrates a major bonus (or flaw, depending on your point of view) of the crossover: a good one can work you into a new fandom effortlessly. Painlessly. Unexpectedly. (I'm proud to report, though, that I've managed to resist the lure of "Homicide." So far.)
Best FF That Proves That It's a Bad Idea to Mess with History If You've Had Too Much to Drink. Well, Especially If You've Had Too Much to Drink.: The Most Beautiful Girl in the World, by daegaer. Good Omens x The Iliad, Paris/Helen, plus assorted hints of slash. I'm still bitter about Troy, and I haven't even seen the movie; all I had to do to get my hate on was read cleolinda's Troy in Fifteen Minutes. I got as far as Achilles in bed with two women and I could feel the rant welling up inside. Just as well I didn't see it, really. In any case, I offer this to those of you also nursing resentment; here is Paris as I always knew he was. And an explanation for the whole mess that's really the only believable one if you insist on discounting the gods. Yes, Wolfgang, I'm talking to you. (Can't wait to see what he's going to do to Ender's Game. Actually, no, I can wait. Forever, if necessary.)