If you only watch one movie to prep, it should be the first Avengers film. (But you should obviously consider watching the other movies on this list as well.) It's the first movie to bring all of the Avengers together and is directed by master storyteller Joss Whedon, which means that the story has a perfect combo of complexity, heart and wit, and it will surprise you at least once.
If you're still on the fence, this clip should convince you (unless you hate everything good):
Captain America: The Winter Soldier (2014)
Another essential. Over the course of the preceding movies, Marvel carefully builds a world, and then, in this movie, COMPLETELY TEARS IT DOWN. A surprise turn of events will leave you gasping in shock.
The Winter Soldier dives unexpectedly into the darker implications of everything set up in the first Avengers. I don’t want to spoil anything, but this is the superhero movie that our modern era deserves, a movie which tangles itself in questions of government surveillance and freedom vs. privacy.
Also, there's intense homoerotic subtext. Try to finish this movie thinking that Captain America is straight. I dare you. There is amazing chemistry between Captain America and Falcon, as witnessed in their lingering, flirtatious glances in this clip:
Other reasons to check this one out: Samuel Jackson is in it, there's a car chase that is simultaneously the funniest and most heart-stopping scene of its kind, and this elevator fight scene:
The Solo Films
Most of the other Marvel offerings are solo films, usually origin stories or character studies. They add nuance and depth to the ensemble Avenger films, but aren't completely necessary to understanding the movies. Here’s a quick guide:
The Thor Series
The Thor movies are solid, fun action flicks, and they are actually my favorite of the solo movies. They aren’t afraid to make fun of Thor, and that keeps them fresh and surprisingly funny. Also, all of the movies explore the relationship Thor has with his sometimes villainous brother Loki, a relationship that is infuriating, complex, violent and tender.
Thor (2011):
The first Thor movie is a great prequel to The Avengers.
If you want to get deep with it, its kind of like a retelling of the events leading up to the second Iraq war. A cocky young ruler decides to end the stalemate his father brokered with a rival country by preemptively attacking it. Only in this retelling, George W. has the universe’s most amazing abs, and he is banished by his father to Earth, where he has to deal with the consequences of his arrogant jingoism.
A sample of the humor that makes these movies great:
Thor 2: Dark World (2013)
As the title suggests, this one is darker in tone, although it still manages to put in some humor. This is the third movie with Thor and his brother Loki. They’ve seen many battles, some against each other, some on the same side. Their ability to simultaneously hate and care for each other makes them one of the most interesting sibling relationships ever portrayed onscreen. They are also the subjects of quite a lot of slash fiction which imagines them as lovers (just sayin').
I think it’s also important to mention (and I say this as a card-carrying lesbian) that Chris Hemsworth’s abs deserve their own film.
Guardians of the Galaxy (2014)
None of these guys will be in Avengers 2, but they will show up in later movies. The movie has an amazing '70s soundtrack, some great characters, and features the best dance scenes in Marvel movie history. See for yourself:
That said, I have major beef with Guardians.
The plot resolution was weak and the romance lacked chemistry and didn’t really make sense. It went something like this:
Him: I use women and don't care about anyone but myself.
Her: I have no time for love and I dislike you.
Him: Now that we’ve spent like 20 minutes together, I love you.
Her: I love you too.
Me: 
I was bummed out by the sexism in the movie. Early on, women are props to establish that the main character is cool. (He beds women and then forgets about them. Neat!)
Then, despite the fact that he is an averagely-powered bandit, when he encounters an intergalactically-known elite assassin, he outmatches her. She obviously becomes his love interest by virtue of being someone who doesn’t like him.
Also, there’s the cliche inspirational speech by Chris Pratt, a.k.a. the guy who is now the leader of the group by virtue of being a white human male. Sigh.
But Guardians isn't a complete loss. It doesn’t matter that the two human(ish) leads are meh because the breakout stars of this film are Groot, a kind-hearted, barely-verbal warrior tree, and Rocket, a crass, hyperactive, genetically-modified raccoon. Their chemistry is out of this world (zing!), but this isn’t some Disney stuff. They are funny, rude, well developed, and kick ass. Even with the problems listed above, they make this movie worthwhile.
Black Widow
HA! Did you think you lived in a world where females can carry an action movie? Oh wait, you do live in that world. Just look at Hunger Games or Divergent. Somehow, the producers at Marvel Studios have their heads too far up their you-know-whats to realize that female superheroes sell. So no Black Widow movies for you. But Iron Man gets three movies...
Black Widow is in the Avengers movies and she plays a big role in Captain America: Winter Soldier, so if you want more of her, that’s your best bet. She also plays a supporting role in Iron Man 2, if you are absolutely desperate for a fix.
Maybe someday we’ll get a whole movie.
Iron Man
Iron Man is like if that mansplaining Libertarian from class was actually as witty as he imagined he was (but was still a jerk) and he also got to punch stuff in a robot suit.
I’m going to be real with you and say that I’m not that into these movies, which I understand is a borderline treasonous statement, since these are the most successful of the solo films. But there it is.
Iron Man (2008)
Standard origin story. Terrible facial hair.
Iron Man 2 (2010)
Classic insufferable Tony Stark. Our first introduction to Black Widow. Some cool fight scenes.
Iron Man 3 (2013)
The most relevant to the new Avengers movie. It's also the most interesting of the three because of the cracks in Tony’s glossy persona.
Captain America
We’ve already established that Winter Soldier is a must-see. But Captain America also has an origins movie.
Captain America: The First Avenger (2011)
This is a quiet, thoughtful origin story, nostalgic for WWII, while also being somewhat aware of the complex forces of a war machine.
Watch it if you love the 1940s or if you want to see more evidence that Captain America is a closeted gay. Never has a superhero been more reluctant to kiss women. How I wish Marvel would commit to it and let him come out already! It would be so powerful and subversive. Until then...
The Hulk (2008)
The origin movie is out of date and occurs before Mark Ruffalo took on the role. So, full disclosure: I didn’t watch it.
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