Pietro likes ladies who in some way resemble his sister. This is canon and he's admitted to it at least once. It has less to do with lolololtwincest (although honestly, it could also be read that way) and more to do with Wanda being his only female role model for a very long time. His definitions of femininity and mothering/nurturing are solidly based on her, so that's what he looks for in a female life partner.

In a guy... He's a lot more non-specific. His best male friends have generally been tall, blond, and built a little bigger than him. I think it's safe to say that that's the type he'd find physically attractive, but personality-wise it's all over the place. He likes people who remain interesting, whether it's through their values and convictions, or just being straight up crazy. As long he can't get bored, it's all good.