As the only two Star Trek shows to share the airwaves and the same quadrant of space, The Next Generation and Deep Space Nine were the two most natural crossover partners. And although there were occasional guest appearances and even a couple characters who moved permanently from one show to the other, we never got the big epic team-up that we all know we deserved.

Indeed, the scene above, from Deep Space Nine's first episode, is the only time Captain Picard and Commander Sisko ever even met. Of course, this scene also makes it pretty clear why there was never a big crossover, and it has everything to do with the fact that Sisko had actually met Picard once before: when he was Locutus of Borg, at the battle that killed Sisko's wife. That kind of history can make a jolly little crossover somewhat more difficult.

Stargate: SG-1 meets Stargate: Atlantis

Now, the other long-running science fiction franchise with multiple incarnations and the word "Star" in the title has been rather better about making with the crossovers. The 2006 SG-1 episode "The Pegasus Project" was the first team-up between two Stargate shows, although it certainly wasn't the last. As you can see in the clip up top, the episode made two things very clear: only the most galaxy-threatening danger would require the two crews to combine forces, and Rodney McKay really is an insufferable bastard, no matter what show he's on. (Note: Not everyone feels this way. You can read our love letter to Rodney McKay here.)

Doctor Who meets The Sarah Jane Adventures and Torchwood

The end of the new Doctor Who's fourth series brought the epic extravaganza/clusterfuck, "The Stolen Earth"/"Journey's End," in which the Doctor teamed up with all his old companions, plus the characters from Torchwood and The Sarah Jane Adventures to accomplish...something. As we observed when it aired, the whole thing felt like Russell T. Davies's attempt to turn self-indulgent, crossover-heavy fan fiction into something approaching high art, with the added bonus that his fan fiction actually got to be the big series finale.

Did it work? (Hint: not really.) It's all way too overwrought, silly, and, yes, self-indulgent for its own good (does anyone really care what Captain Jack thinks about Sarah Jane's Slitheen-fighting skills?), and Tenth Doctor's guest appearance on The Sarah Jane Adventures is a far more contained, effective crossover. (We've also got high hopes for the Eleventh Doctor's upcoming appearance on Sarah Jane's spin-off.) Even so, much as I don't really care for the result, I've got to admire the lunatic ambition of Russell T. Davies. Everytime I think he can't top himself in crazy, he comes up with...well, this.

The Six Million Dollar Man meets The Bionic Woman

Although The Bionic Woman quickly emerged as its own show independent of Steve Austin's continuing adventures, the two superpowered do-gooders often teamed up when the task was just too difficult for either one of them to do it alone. One of the most famous team-ups, the three-parter "Kill Oscar!", placed murderous fembots squarely in the popular consciousness, where they've remained ever since. But the video we have here is from "The Return of Sasquatch", in which the two team up to defeat leisure-suited villains who are mind-controlling Sasquatch in order to kill some benevolent aliens. Or something. Point is, the 1970s were awesome.

The X-Files meets Millennium

Chris Carter's other show about FBI agents and paranormal conspiracies got canceled before it could wrap up its big mysteries about a Christian sect turned shadowy law enforcement firm, that believes the millennium will bring about the end of the world...with a little help. Of course, that sort of mystery is tailor-made for an X-Files episode, so Carter brought the worlds together for an X-Files episode that doubled as a de facto Millennium series finale. Lance Henriksen returns as a now broken Frank Black, the FBI criminal profiler who was chasing the conspiracy in Millennium. The last couple of years have pushed him to the brink, but with a little help from Mulder and Scully, he helps them with their investigations and shows just how brilliant he was at his job, as you can see here.

Batman meets Superman in "World's Finest"

Bruce Timm's epic animated franchise remains the gold standard for televisions superheroes, and that likely won't change anytime soon. That said, the series did undergo a seismic shift from the more grounded, heavily art deco feel of the original Batman: The Animated Series to the cosmos-spanning, visually streamlined Justice League shows. The three-part Superman: The Animated Series episode "World's Finest", which features the first team-up between the Man of Steel and the Caped Crusader, isn't just a crossover for the two characters - it's the de facto crossover point for the two phases of the DC animated universe.

That's all rather less interesting than the actual story of "World's Finest", which spins the hoary old Batman/Superman team-up in brilliant, twisted directions. Batman and Superman team up, sure, but they don't really trust each other and they definitely don't like each other. Lex Luthor enlists the aid of the Joker, and the story soon becomes as much about the two villains fighting each other as their mutual foes (not to mention the big smackdowns between their henchwomen, Mercy Graves and Harley Quinn). And, in perhaps the most devilishly clever innovation, Lois Lane falls madly in love with...Bruce Wayne.

Batman meets The Green Hornet

From the sublime to the ridiculous, here's the Adam West version of the Dark Knight (not that that nickname really fit this particular Batman) meeting up with the other iconic superheroes of sixties TV, the Green Hornet and Kato. Sadly, we don't get the big Adam West/Bruce Lee smackdown that probably would have represented the apex of civilization, but it's fascinating to watch these clips as a reminder of how much has changed for these two franchises between these TV shows and their current movie franchises.

Consider - here, Batman is the campy buffoon, and the Green Hornet and Kato the serious, borderline gritty crimefighters. It's pretty much impossible to imagine Christian Bale and Seth Rogen teaming up like this, and doubly impossible to imagine it unfolding anything like what we see here. Although some things never change - Batman was a highly rated TV show that quickly latched onto the popular consciousness, while The Green Hornet, Bruce Lee's rising star notwithstanding, failed to catch on and lasted only a single, poorly rated season.

The Jetsons meet the Flintstones

Admittedly, this 1987 TV-movie came long, long after the shows' 1960s heydays, but for for the sheer, giddy thrill of mashing together two similar but seemingly unrelated franchises, The Jetsons meet the Flintstones remains the gold standard of shameless TV crossovers. (Something that Bart Simpson alluded to in his own show's notorious crossover with The Critic.)

I remember this movie felt pretty silly even as a kid, but it does do at least one thing right - the focus remains squarely on the Jetsons adjusting to life in Bedrock (and, later, the Flintstones to life in the future), meaning there's always good material to explore what makes the shows similar and what makes them different. If nothing else, Astro is just as good at running over Fred as Dino is, as we see above.

The Simpsons meet The Prisoner

Am I cheating a bit by including The Simpsons? Maybe. Certainly, no one would argue that the guest appearance by the casts of Dragnet or The X-Files mean that those shows take place in the same fictional universe as The Simpsons, but still...this is something special. "The Computer Wore Menace Shoes" is one of the last great Simpsons episode, an enjoyable episode that's sort of about blogging until the craziest third act swerve in television history: Homer ends up on the Island, a clearly updated version of the Village, complete with a long-term prisoner designated Number Six.

Considering that the legendarily prickly Patrick MacGoohan actually agreed to appear in the episode (and, judging by his performance, was having a good time doing it), I really can't ignore this loving tribute to The Prisoner in our list of great crossovers. Besides, it's important that we all learn how to defeat Rover (or, as Homer more accurately calls it, "an anti-escape orb!"). Sure, it's a little out there, but I'm just getting warmed up for our final crossover, namely...

Power Rangers in Space meets The Teenage Mutant Ninja Turtles

Yes, you read that right. In what has to remain the single strangest crossover in (vaguely) science fiction TV history, the fourth incarnation of the unkillable Power Rangers franchise (18 years and counting!) teamed up with the live-action version of the Ninja Turtles. It's just as cheesy and ludicrous as it sounds, as that clip up top will demonstrate to anyone brave enough to push play. I'm not sure what intelligent commentary I can offer to make any of this make any sense, so instead I'll just offer this as a confession: I'm pretty sure I watched this when it first aired, and I may even have sort of enjoyed it. In my defense, I was nine at the time. But thirteen years later, this crossover is still burned into my brain as the most ridiculous thing I can't quite believe actually exists.