When Don met Betty, he was drawn to her sweetness and Grace Kelly-like beauty. In a flashback, he raved to Anna that he had met a beautiful, happy girl and he loved the way she laughed and the way she looked at him.
When he met Rachel, he fell in love with her intelligence and the way she challenged him. Sure, she was an attractive woman too, but what Don seemed to respond to the most was the way she talked, the things she said, and the fact that she, like him, was something of an outsider.
With Megan, he saw some of the same qualities he saw in the young Betty (charm, innocence, beauty), and also that she would make a great stepmother for his children.
With Sylvia…it’s a bit complicated. What exactly did he see in Sylvia? The first time we saw them together, he told her he wanted to end things. “I want to stop doing this,” he said. But the lure of her was apparently too great to give up. Throughout season 6, she remained the object of his strongest desires. Sylvia was pretty (although not as model-beautiful as Megan), she seemed smart, and despite having an affair, she had a strong sense of morality and guilt.
Betty, Rachel, Megan, and Sylvia. Arguably these have been the main women in his life. (Not counting Anna and Peggy, women he had platonic relationships with). Faye, Bobbi, Midge, Suzanne, and others were important to him for a while, but I would argue not as important. He never discussed marriage with them, or running away with them, and he never seemed in any way obsessed with them.
So what does Don want? Based on his history, he wants a sweet, model-beautiful woman to marry, but the dark, complex women like Rachel and Sylvia are the ones that get under his skin and make him feel understood. He loved Betty when he met her, but after they were married, she often wondered, “Who’s in there?” Megan knows more about his past than Betty did when he married her, but he still often keeps her in the dark about things. It is often hard to tell just how much of himself Don is capable of sharing with one woman. He can share parts of himself, but not all.
If Don divorces again, will he ever find “everything” he wants in just one woman? The sweet docile beauty of a Betty/Megan combined with a tough, complicated personality like Rachel/Sylvia. Or maybe he doesn’t really want it all in one woman. Maybe he will always seek out a certain type of woman to marry, and another type of woman to have an affair with.