...at least, if you are into fan fictions and Harry Potter.
I love fan fictions, and I'll shamelessly admit that I like Harry Potter too, but I usually avoid crossovers. That is because however interesting and exciting is - in theory - to have your favourite characters from different fandoms together, it often ends bad.
So, it's a public duty of every fanfic reader to recommend good crossovers, if they stumble upon them. Here are some I found recently (all the quotes are from the stories' summaries).
Not Quite A Maia (Harry Potter x Lord of the Rings)
"Middle Earth has a problem: Gandalf the Grey is AWOL in Time & Space after destroying the Balrog of Khazad-dûm. But who will take his place in the Quest to defeat the Dark Lord Sauron? Not who you think..."
This is a very well written fic, and one of the best HP/LOTR crossover I've ever read. Bonus points for not featuring main characters of HP-verse (someone else will have to save the day in Middle Earth, not the usual Golden Trio). And if there are some minor glitches in the plot (especially in the last chapters), the author has undoubtedly a knack for hilarious situations.
If you can read through Augusta's chapters without laughing out loud you are not human. Really.
Evil Be Thou My Good (Harry Potter x Hellraiser)
"Nine years ago Vernon Dursley brought home a certain puzzle box. His nephew managed to open it, changing his destiny. Now, in the midst of Voldemort's second rise, Harry Potter has decided to recreate the Lament Configuration... and open it... again."
Even if I am a notorious addicted to horror and b-movies (and more so to horror b-movies) I've never seen Hellraiser.
But you don't have to, to enjoy this story.
It's well written, it explains the backstory for the Hellraiser universe (or at least what you need to know about it) and features a refreshing Dark!Harry. The only thing I didn't like is a slightly OOC (Out Of Character) Voldemort, who's described in canon as intelligent and cunning and here seems to me a bit gullible. But apart from that, the story is worth a read.
That Which Holds The Image (Harry Potter x Doctor Who).
"Harry Potter faces a boggart that doesn't turn into a Dementor or even Voldermort, but into a horror from his childhood. Now the boggart isn't even a boggart anymore. There's no imitation. That which holds the image of an Angel, becomes itself an Angel."
I have a soft spot for the Weeping Angels. They are pure evil genius.
If you don't know what I'm talking about stop reading now and go watch Doctor Who episode 3x10 "Blink". It's really a masterpiece.
So Weeping Angels in Potterverse would be awesome even if the plot of this story weren't as good as actually is. The only thing to make it better would be to have 10th Doctor instead of 11th. But then, you can't have everything, I guess.
If you read any of these and want to talk about it or if you have more crossovers to recommend, drop me a mail.
For more fan fiction recommendations, I suggest you to check this detailed index on tvtropes.org.