Breaking Bad and the Simpsons would be funny!
But for me it would be Xena/Red Sonja crossover or a Batman/Breaking Bad crossover or a Supernatural/Hellboy crossover or a Doctor Who/Avatar crossover or a Hercules: The Legendary Journeys/Thor crossover.
I have a list of this so this might take a while...
I think Matt Fraction's non-answer was priceless, but the best answer has to go to Jeff Parker though. My only problem with having Sherlock and Detective Chimp crossover, would be that Benedict Cumberbatch would actually make the perfect Detective Chimp. Aw, shucks, he can play both roles!
@stormmagician: I would love a Supernatural/Hellblazer crossover so when Castiel and Constantine ever meet they would be like "why are you wearing the same clothes as mine?!" And Dean and Sam would look at them so awkwardly. lol
I always wanted my dream crossover to be my future comic/TV series , CD (I'm putting initals because I haven't copyrighted and I don't want the name to be stolen) Vs anything I ever loved like Marvel, DC, Capcom, SNK , Ninja Turtles, Avatar, etc.
My Little Pony/Breaking Bad would be awesome because they would be high on rainbows. lmfao
Breaking Bad & The Walking Dead, the setting and scenery is similar.
Since they're both AMC shows and The Walking Dead had Heisenberg's Blue Sky Meth as a cameo, it's possible they're set in the same universe, just different times.
Todd MacFarlane, Erik Larson, or Jim Lee on The Big Bang Theory would be cool, and Agents of S.H.I.E.L.D. certainly has opened the doors to a lot of Marvel characters making TV debuts, we had a reference to Spider-Man (though until Sony lets those rights go, we won't see him in a good live-action role that is true to Spider-Man) of course Agents of S.H.I.E.L.D. has also seemed to have the running gag of close to but not really the hero you think someone is as Centipede super-humans, we already have seen the close-to-but-not Luke Cage and Sunfire on that show.
Game of Thrones/Thor
Venture Bros/Wolverine so we can get Brock Samson vs Logan.
Samurai Champloo/Shang Chi
Iron Fist/ WMAC Masters
Ultimate Spider-Man Young Justice
Tomb of Dracula/Vampire Diares
Werewolf By Night/Teen Wolf
Angel and Faith/True Blood
Teen Titans/X-Men Evolution
Sabertooth/Man vs Wild
Kraven The Hunter/Survivor Man
The Planet of Tatooine/Duel Survival