For years Thanos has just been the guy hanging out on the sidelines in the Marvel Cinematic Universe, leaving others to do his dirty work. Next year, Avengers: Infinity War will finally show what the Mad Titan is capable of. It will take the Avengers, Guardians of the Galaxy and more to stop him, and make no mistake, it will be the biggest conflict the MCU has ever seen, potentially extending into Avengers 4. That said, these are superhero tales, and even with the inevitable loss of life, Thanos will be defeat. The question is who will be the one to take him down.
Avengers: Infinity War will be packed with a lot of superheroes, but only a select few of them could successfully kill him. Keeping in mind Thanos' natural abilities, how he may successfully manipulate some or all of the Infinity Stones (hence why we can't include The Vision here) and who's been confirmed for the Marvel blockbuster, here's who we've ranked from least likely to most likely to get the job done, whether due to their power levels or for narrative reasons.
Warning: some spoilers for Guardians of the Galaxy Vol. 2 are ahead!
9. Iron Man
Let me preface this by saying that Tony Stark wouldn't stand a chance going toe to toe with Thanos while wearing one of his typical armored suits. Even without the Infinity Stones, Thanos is one of the strongest beings in the universe, and he could easily rip apart Tony's trademark weapons. The only reason Iron Man is being included on this list is if one of two scenarios happen in Avengers: Infinity War. The first is if during the movie, Tony comes across some kind of alien technology that's able to boost one of his suit's power to levels that couldn't have been reached even using cutting edge Earth technology. The second, and more preferred scenario, is if Tony is somehow able to control one of the Infinity Stones and use its power against Thanos. If neither of those things happen, then chances are extremely slim that Tony will be the one to kill the Mad Titan.
8. Doctor Strange
Unlike the Avengers and the Guardians of the Galaxy, Stephen Strange harnesses magic to keep our reality safe, so maybe magic is the key to stopping Thanos once and for all. The Infinity Stones are arguably the most powerful cosmic forces in the galaxy, but with the right spells and strategy, Doctor Strange could conceivably overpower Thanos. Spell-casting is tricky, but given what Doctor Strange was able to accomplish in his first movie, and all the added experience he'll have gained between then and Infinity War, he could be up to the task. On the off chance that magic isn't enough, don't forget that Strange also has experience wielding the Time Stone. So even if Thanos gets his hands on some of the other Infinity Stones, maybe the sorcery/Time Stone combination could give Strange an advantage.
Thor is going to be put through the ringer later this year in Thor: Ragnarok, which will include losing his trusty hammer, Mjolnir. However, as revealed in concept art, the God of Thunder will be ready for battle again in Avengers: Infinity War, this time armed with a new weapon. While he may not be quite at Thanos' level of power, Thor does come close both through his natural Asgardians abilities and his own particular set of skills. That's not to say that his fight against Thanos wouldn't be the toughest of his life, but assuming that Thanos hasn't become invincible from the Infinity Stones and is relying on his natural might, Thor could find an opportunity to strike a mortal blow, even if it means having to sacrifice himself.
Loki struck an alliance with Thanos in The Avengers, but the God of Mischief failed to keep up his end of the bargain when he didn't return the Tesseract to the Mad Titan. It's not Loki's fault, a team of superheroes got in his way! Regardless, several years have passed since the Battle of New York, and we wouldn't be surprised if Loki still holds a grudge against Thanos for leaving him behind. It's unclear if Loki will change for the better in Thor: Ragnarok, but even if he doesn't, you can bet that if sees an opportunity to kill Thanos, he'll take it. Loki isn't physically imposing, so in a direct conflict with Thanos, he'd likely lose. However, the reason Loki is one of the MCU's most dangerous villains is because of his illusions and scheming mind. If he kills Thanos, it will be through one of his "tricks," and the Mad Titan may not even die realizing that Loki was the one who slayed him.
As we've said several times already, Thanos is powerful without the Infinity Stones, and one of the few beings who can match him physically is The Hulk. Rest assured, Avengers: Infinity War will be packed with a lot of action, but few, if any, of those moments would measure up with seeing Hulk and Thanos engaged in fisticuffs. Whether he's on Earth or in the Sakaar gladiator ring, Hulk has built quite the reputation across the galaxy, so it's not unimaginable that Thanos would welcome the chance to deliver a beating to the Green Goliath. Admittedly, Thanos has defeated Hulk in the comics, but Infinity War doesn't have to follow suit. Just imagine seeing a Hulk vs. Thanos fight ending with the former beating the latter into a pulp. Now that's great superhero cinema.
After killing her family when she was a child, Thanos claimed Gamora as his "daughter" and raised to be one of the galaxy's most proficient assassins. While she's not as hellbent on revenge against her "father" as Nebula (more on her later), Gamora certainly holds a grudge against Thanos, and if given the opportunity, we doubt that she'd pass up slicing his head off or obliterating him with a giant laser gun. Gamora is on the heroic path now that she's part of the Guardians of the Galaxy, but given her past with Thanos, as well as how all of existence could be destroyed should he succeed, she'll kill her "dad," though the aftermath of that decision would almost certainly be explored in Guardians of the Galaxy Vol. 3.
3. Thanos Himself
There are two main ways this idea could be looked at with regard to Thanos. The first is the traditional approach where the villain is responsible for their own demise, usually through overconfidence or being deceived by the story's protagonist. But the second option is far more interesting, and it's focused on Thanos' obsession with Death. Like his comic book counterpart, it's implied that MCU's Thanos will try to court the personification of Death, although it's unclear whether this be an entity we haven't met yet or if this franchise is using Cate Blanchett's Hela (the Norse goddess of death) as a substitute. Regardless, if the Infinity Gauntlet story is being used for inspiration, then the Mad Titan will seek out the Infinity Stones to wipe out billions of beings so he can prove his love to Death. Maybe before he can do that, or after such changes to reality have been reverted, he'll be put in a situation where he willingly accepts Death's cold embrace, thus fulfilling his long-held desire and ending his reign of terror.
Like Gamora, Nebula was raised as Thanos' daughter, but because she lost many of the challenges that he put his "kids" through, she endured more abuse and torture from. Following Guardians of the Galaxy Vol. 2, Nebula and Gamora seem to have repaired their relationship, but now all of Nebula's hatred is directed towards Thanos. The only reason she didn't permanently join the Guardians of the Galaxy is because she's determined to kill the Mad Titan. Having either of Thanos' daughters kill him would be fitting, but Nebula is more deserving of the opportunity given that Thanos not only abused her, but replaced most of her body parts to make her "better." Watching her dish out revenge would be immensely satisfying. It's also worth noting that in The Infinity Gauntlet, Nebula played an integral role in undoing all the damage Thanos caused. Maybe the movie can follow a similar path by having her be the one to eliminate him.
Nebula isn't the only one who wants to kill Thanos. In Guardians of the Galaxy, Drax the Destroyer aided in killing Ronan the Accuser, the Kree fanatic who killed his wife and daughter. However, realizing that Ronan was just a "pawn," Drax told Gamora that he needed to kill Thanos to truly avenge his family. Drax's didn't discuss his desire for vengeance in Guardians of the Galaxy Vol. 2, but if/when he crosses paths with the Mad Titan in Avengers: Infinity War, you can be sure he'll pull out his blades and try to decapitate him. Of course, Drax has habit of charging into battle without thinking, so his first attempt at Thanos' life may not go well. But later on, when Thanos is weakened and can't rely on the Infinity Stones, Drax can finally fulfill his mission and eliminate the alien warlord responsible for so much of his pain.