After taking a semester of noir literature, I know a femme fatale when I see one. Yes, Natasha is more than a pretty face, but she knows how to use her sexuality to her advantage. That aside, she is a stunning example of a Slytherin (and proves that not all Slytherins are evil, so double point - ha!)

So let’s look at her character:

1. She’s a guarded character, and doesn’t have clearly emotional reactions like Tony and Steve. She’s slower to reveal what she’s thinking and feeling. Yes, she’s a spy, but she has grown up in that lifestyle, which has consequently shaped who she is. She keeps her feelings in check and under lock, and focuses on the getting the job done. She believes the end justifies the means, such as her stance on ‘Phase 2’ in the movie, or bringing in Banner.
2. Duplicity is a major part of her character. And yes, once again, the spy card can be played, but just like before being raised as a spy has caused certain characteristics to become engrained in her nature. I think this is best shown when she goes to Loki. She plays a part (similar to when she was interrogating the Russians at the beginning), the part she knows he wants and expects to see from her (because he expects and enjoys the weakness of humans). But she just as easily slips back into her calm demeanor once the information she needs has been gathered. In Iron Man 2, we see she is capable of adapting in social situations to the extent where she gains Pepper’s trust. It’s a skill that can be used for ill, as Loki displays, but Natasha puts it toward what she sees as redemption.
3. I think one of the biggest parts of Natasha’s character is also one of the hardest to understand. As I already mentioned, she doesn’t put much value in emotions (i.e. “love is for children”), but she has a clear idea of what value people hold to her, especially if she owes them. This screams Slytherin to me, and I don’t mean she only values people to be of use to her. She values them, and thus they are valuable. And when what she values is messed with is when she reacts in a more volatile manner. This scene with Clint shows this:
   Clint: You’re a spy, not a soldier. Now you want to wade into a war. Why? What did Loki do to you?
   Natasha: He didn’t…
   And yes, to an extent Loki did attack her with cruel words and personal details (ones she’d probably be happier forgetting), but they were relatively harmless in the grand scheme - Natasha knew she was besting him in the end, so she wouldn’t be too sore afterwords. But Clint was a different matter, a matter significant enough she acted out of the norm. 

All in all I think Natasha is a complex character who has a dark past, and is trying to overcome that. She is determined to prove herself a different person, a good person, but she still remains cunning, ambitious. Yet she is a good person, and a heroine. She might not have the emotional and virtuous will to save humanity like dear Captain America (she wants to clear her ledger, remember), but that makes her efforts no less meaningful than his. Self-preservation is a common trait from a Slytherin, but that just makes her choices and sacrifices that much more meaningful in the end.  

In my mind, Tony Stark is a rather quintessential Gryffindor. Yes, he is brilliant…and yes, he often seems to lack a moral compass - however, as we see in Harry Potter, most characters often display the characteristics of multiple houses.

Let’s Look at Tony’s main characteristics:

1. He’s an attention seeker. While Ravenclaws are smart, and we all know Tony is a genius, Stark wants his genius to be acknowledged. A lot of Ravenclaws also tend to be introverted, and clearly Tony is an extrovert. But Ravenclaws don’t need the validation from others like Tony does to be satisfied with their work. 
2. He copes/reacts with humor. Whether a situation is too dangerous or too serious, Tony stills cracks the joke. If he was a true Ravenclaw, or even a Slytherin, I think he would have far more decorum and willingness to adapt to the situation socially. Instead, he reacts with humor that is often not fitting to the situation, making the snarky comments or innuendos.
3. He is a daredevil and adrenaline junky. I think we can agree that Ravenclaws and Slytherins rarely do things on impulse (Ravenclaws are rational, and Slytherins care about self-preservation). Tony risked his life just flying in the suit for the first time, and has many other avenues available to him to right the doings of Stark Industries without risking his life. In Iron Man 2, he handed his potential death with even riskier behavior instead of hunting for a solution. He drives the fastest cars, finds the tallest rollercoaster, and takes the biggest risk.

Ok, since he’s often put in Ravenclaw, I’ll explain why I think he ISN’T one. Ravenclaws would not be as impulsive as Tony is, and I believe they would find a different route than becoming a vigilante to make up for Stark Industry’s wrongdoings. As I said before, he uses his intelligence quite often, but he seems to get a rush from more than just the creative process itself, but it’s display. He’s more proud of upshowing others with his intelligence than the discoveries his intelligence has made. That’s why he’s done so well in the business world. He needs others, however, to acknowledge his genius to validate it to himself. 

Also, I think there is a scene that clearly shows he’s a Gryffindor, when Thor first appears and takes Loki away:
Steve: Stark, we need a plan of attack!
Tony: I have a plan: attack.
I think any Ravenclaw would want to create a strategy before going in. That’s not to say that Gryffindors can’t come up with a strategy, they’re just more comfortable going in without one if the situation calls for it.

Ok, I’ll stop with the mega-rant. But that’s basically a summary of why I think Tony is a Gryffindor. Feel free to comment/argue :)