The Fantastic Four are a unique bunch in comic history. The first collaboration of Stan Lee and Jack Kirby, the team's debut is generally regarded as the birth of Marvel Comics, and the juggernaut that would follow, and yet the FF really isn't that fantastic. I've never been able to get my arms around Reed Richards and his aloof brilliance (I prefer his nemesis Doom), to warm up to the juvenile antics of the Human Torch (Spider-Man is a better boy hero), and while the Thing enjoys some decent characterizations, he's never really rocked my world. They're a dysfunctional group, a sort of prolonged ego experiment for Mister Fantastic, a family business as much as a super-team. I've always found the best part of the FF to be Susan Storm, girl and woman.
The Invisible Woman never really wanted to be a heroine. She just sort of got swept up in the backdraft of Reed Richards' hubris, following her heart into accidental superpowers and a lifetime of tribulation. Initially she was the weakest member of the team, but as time went on her skill and power grew. Still, what made her in some ways the most formidable of the Four was her indomitable will. She was at once fiercer and gentler, more passionate and more wise than her relatives/teammates, and despite her depictions in the early 1960s as a fragile female, a scatterbrain or ditz, Sue swiftly became guiding light and den mother to the bunch. They tried to dial up the sex kitten angle with peekaboo costumes and weak cheesecake, but it never took, not even with the starlet miscasting of Jessica Alba or Kate Mara. She might have been tempted by Namor, but her mutual love with Reed has been one of Marvel's foundations for half a century.
She became a mother, too, a successful one in an environment where quality parenting is almost unknown. Let's face it, Cyclops might be the worst parent of all time, Scarlet Witch and Vision had imaginary kids, and Magneto's three children have all essentially tried to kill him their entire adult lives. In light of the weak parenting record of Marvel's hero roster (hat tip to Luke Cage, who seems to be managing), Sue's devotion to Franklin and Valeria, her insistence that they deserve some stability in the nutty world of super-capers, is both classic and cutting-edge. She puts her family first, whether that's her husband and brother and best friend or her children. She saves the world in her work clothes, and then saves it one child at a time in the Baxter Building, regardless of the cost. She might be the most heroic one of them all.