One of the biggest plot points throughout Mad Men is Don Draper's infidelity and his constant romances. It's very rare that there's a period where Don isn't involved with someone, and more often than not he is actually involved with multiple people, often cheating during his marriages.
It's an addiction for Don, who gets a real thrill from it, but over the course of the series, he is romantically involved with a real array of women. Don doesn't tend to be very picky, which means he ends up with women from totally different walks of life, and that brings a varying amount of intelligence between them all.
Allison
Allison is someone that Don has a very brief fling with, and she is someone who has one of the biggest reactions of anger and sadness when he calls it off. Allison works as his secretary, but that is a job she struggles with, often making mistakes or taking her time with things, which is due to her lack of intelligence.
While she's certainly a fun personality and someone who clearly cares a lot, there is no doubt that Allison isn't one of the smartest characters to appear on the show, which seems to be something that he takes advantage of.
Betty Draper
Don's wife for a large chunk of the show is obviously an incredibly important character within Mad Men. However, Betty Draper is not one of the most intelligent, and she is someone who doesn't understand a lot of complex issues and struggles to see a lot of obvious things happening in front of her, such as Don's affairs.
Betty is very childlike with her behavior, with her temper and lack of patience being examples of that. However, it is very evident throughout the series that Betty has some form of mental health issues, which is why she attaches herself to her child psychologist, even though it's not meant to be for her. Just before her death, Betty does actively attempt to improve her intelligence, showcasing that she at least wants to learn more.
Diana Bauer
Diana Bauer was one of the least popular characters in the show, with this storyline being one that many people didn't enjoy. She is a waitress when Don meets her and is shown to be someone who isn't that intelligent throughout her run on the show.
While Diana is inquisitive and does read, she never really engages much in conversation in the way that many of Don's partners do. This relationship is seen as Don hitting rock bottom in the show, as it isn't long after that he heads to the retreat.
Midge Daniels
Midge Daniels is the first woman that Don Draper is shown having a relationship with that isn't his wife, giving an indication of the type of man he is. Midge is one of the more interesting choices that Don makes, as she is addicted to drugs, living in a poor area, and has another partner who is well aware of Don.
It's a strange setup but clearly done so the news never gets to Betty. Midge can be very intelligent at points, and she has some intriguing views on the world from her perspective. However, there's no doubt that constant drug abuse plays an impact on her intelligence, as there are many points where it is clear she isn't that smart.
Megan Draper
After his divorce from Betty, it doesn't take Don too long to get married again, this time to Megan, who is a bright character from the show. She's much younger than Don, and because of that, she does have a slight lack of life experience at points, which does show.
However, there's no doubt that Megan is a smart character, which is why she's able to excel at everything she puts her mind to. Megan is able to have great conversations with everyone, no matter the setting, and she is always looking to create a fun vibrant energy, which adds to her personality as well.
Sylvia Rosen
The relationship that Don Draper has with Sylvia Rosen is one that ultimately ruins his relationship with his daughter, Sally, forever when she catches them together. However, she is someone that Don does have a deep connection with, and she is also one of the more intelligent people that he ends up in a relationship with.
Sylvia is able to have very engaging conversations with everyone, and her husband being a doctor leads to a lot of interesting conversations when they're all together. She is certainly a smart character and is someone who understands the situation they're in together and the dangers of that.
Bobbie Barrett
Don Draper can't help himself when it comes to Bobbie Barrett, seemingly only getting romantically involved with her to spite Jimmy Barrett, her husband. Ultimately, this comes back to bite him as Jimmy is the one who alerts Betty Draper to Don's affairs.
However, when it comes to the intelligence factor, Bobbie is certainly a very smart individual. She is someone who clearly has a lot of experience and understands the business world very well, always managing to get what she wants because of that.
Suzanne Ferrell
Suzanne Ferrell is an elementary school teacher and someone who showcases kindness and patience throughout her spell on the show. Her charming personality works well for Don, and the two of them hit it off, creating a strong bond that almost leads to him committing to her.
She proves herself to be very smart in how she treats the children in her class, with creative and engaging lessons. While she also looks after Don and seems to be one of the few women in his life that can see he is hurting and not fully happy, which shows a smart side once again.
Rachel Menken
Rachel Menken is one of the most important love interests in Don Draper's life, and someone who really sticks in his head throughout the entire show. She's an incredibly smart, business-savvy individual who is the head of Menken's Department Store, and she had no problem standing up to Don and the rest of his team during business meetings.
Rachel is able to really understand and analyze the work they're doing for her, as she tries to take her business to the next level, which shows great intelligence. Plus, she is also intelligent enough to know when to turn down Don's advances, as she doesn't simply want to be 'another woman' as so many of his romantic interests end up becoming, which is why she sticks in his memory.
Faye Miller
Out of all Don's love interests, Faye Miller is certainly the most intelligent. Because of her job, as a strategist for a consumer research company, she is very good at analyzing people and the manner in which they behave, which is why she's able to get a clear understanding of Don.
The two of them have a really good connection, which is largely down to how smart Faye is. She is able to really see Don's emotional side and work him out, and when he ends their relationship, she understands it's because of his short attention span. While she's hurt by it, she's one of the only women that is smart enough to really understand what is going on with him.