We’re with Quagmire on this one. Brian is terrible. Stewie talks a lot about world domination and killing his mother, but these days, it’s mostly just talk. Penelope, on the other hand ...
For Brian, it's that he faults at them; Stewie trips to the 1960s where he meets a pretty girl, but she's not just any girl; Quagmire's viewed as a woman; and so many more romantic flings happen.