So, here's my team to defend the Earth in the most dire circumstances, drawn from all fandoms.
Team Captain: Captain Titus (Space Marine)
Of an entire race of genetically-engineered super-soldiers who defend the human race from the worst scourges of the Galaxy, Captain Titus of the Ultramarines 2nd Company is one of the baddest. As a Space Marine, he is used to fighting battles in many environments against truly horrific enemies. He is a master of the tactics of legendary Ultramarines founder Roboute Guilliman and he is a capable and skilled battlefield commander. Titus is an expert marksman with every weapon in the Space Marine arsenal; Bolter, Storm Bolter, Meltagun, Plasma Gun, Lascannon, Plasma Cannon, Heavy Bolter and he mastered the Graia-pattern Vengeance Launcher shortly after picking it up. He's also an unmatched close combat fighter, equally capable with the Chainsword, Power Axe or Thunder Hammer. While fending off the million Ork WAAGH! as well as the Chaos Invasion of Forgeworld Graia, Titus slaughtered countless Orks and Daemons with ease. He bested an Ork Warboss in single combat and beat a Daemon Prince to death with his bare hands while free-falling from about 50,000 feet. Titus could probably handle this invasion alone.
First Draft Pick: Black Adam (DC Comics)
Black Adam is as powerful as Superman, isn't weak to magic and isn't allergic to green rocks. He's also willing to kill, which is why I'm taking him over the Man of Steel. I don't want to try and find a peaceful solution, I don't want to negotiate, I want this threat handled with extreme prejudice. Black Adam is ruthless, brilliant, and has the powers of the Egyptian Pantheon. At one point he fought the Justice League, Justice Society, Great Ten, and the Teen Titans simultaneously and won. Read that again. That means he beat Superman, Wonder Woman, Batman, Martian Manhunter, The Flash, Power Girl, Superboy, Wonder Girl, Kid Flash, Starfire, and a bunch of others...all at once. I'm not worried about Adam back-stabbing the team, because he considers himself an honorable man and he has worked with heroes (including his nemesis Captain Marvel) in the face of greater threats before. Recently he became even more powerful by absorbing the power of Isis which enhances his own powers and gives him power over nature. So now he not only has the powers of Superman; he has the powers of Storm of the X-Men.
Second Draft Pick: Nova Terra (Starcraft, Heroes of the Storm)
In addition to brute force, wars are won with stealth and intelligence gathering. This is the purpose of the Terran Ghost program and the best of the lot is November Anabella "Nova" Terra. A powerful psychic and telekinetic, Nova is well-versed in covert operations. Nova is a formidable hand-to-hand combatant and an unparalleled sniper; having received brutal special forces training geared to taking on brutal alien races like the Protoss and the Zerg. Additionally Nova is one of the most powerful psychics in the Starcraft Universe. She uses her psychic powers to render herself invisible, read the minds of her enemies to predict their actions, make herself faster and stronger, and grant herself superhuman levels of endurance. Her telekinesis is powerful enough to bring down sky scrapers (which, on Starcraft's Earth have evolved to be thousands of stories high, dwarfing the mighty Sears Tower easily). She also is capable of unleashing a psionic wave which can kill every living thing for two miles around her. Added to that, she can fire just about any weapon she lays her hands on with expert accuracy and she can hack just about any computer system, including alien computer systems.
Third Draft Pick: The Eleventh Doctor (Doctor Who)
The intellect of a Time Lord. The ability to travel in time and space. The man who’s won thousands of battles against the most deadly creatures in the universe. I’m taking the Eleventh Doctor because he’s got the best balance of The Doctor’s best qualities. He’s brilliant. He’s pragmatic, willing to take life if necessary. He’s a master tactician who can come up with a strategy to completely change the tide of the battle in seconds. He’s free of the guilt which plagued the Ninth Doctor, and he isn’t bound by the sentimental bonds that led The Tenth Doctor to be merciful (sometimes) to a fault. He knows he didn’t kill his people in the Time War, and this allows him to embrace his role more easily. The Doctor is the most feared warrior in the Universe, entire battle fleets turn and run when they hear his name. He’s battled the most powerful military races in the universe and won time and again. He’s outwitted supreme intellects and quantum-level artificial intelligences. With his intellect and ability you might think he should be the leader, but The Doctor traditionally balks when given leadership roles. He doesn’t like to command armies, he likes to be an outlier and attack problems from a different perspective. He will work with others when he needs to, and I think he could balance the other three personalities.