I am not downplaying Janet Jackson's accomplishments, but she was never in the same league as Madonna, who tapped into alot of cultural myth to extend her influence into everyday awareness. That is something most of her wannabees and successors never "got"/"get". It isn't just the youth that you want- Madonna blanketed her fame over the whole genrational board. She ran the gamut of pop culture - 50s, 60s, 70s - and brought not only kids int her sphere, but their parents and grandparents as well. I remember sitting in a Dairy Queen in the early 90s and my friend and I laughed as two old people in their 60s/70s couldn't help but move to the tune of Vogue while it played over the radio - a song that paid homage to stars that were their generation. Years before that, my parents got a kick out Madonna doing old Hollywood Monroe with Material Girl or later Kim Novak in Papa Don't Preach.
No matter how "risque" she was, she still managed to maintain an innocence to her music that when all was said and done, it was infectious and instantly recognizable, becoming a part of the scene and helping to define that scene. "Crazy For You" became the eternal prom song; "Into the Groove" the eternal sappy dance tune with a safe edge. Not alot of grandparents today know any of Britney's songs, let alone Gaga's.
People take it for granted to be blasphemous and iconoclastic these days (in fact, it is the norm and has lost its shock value), but Madonna's "Like A Virgin" unsettled alot of people in the 80s because of its blatant parody of her own name with the Holy Virgin of the Catholic Church. That kind of blasphemy was always held in check on the fringe, the underground (Prince was an acquired taste who wasn't that well-known to Moms and Dads). Madonna brought it into the mainstream and turned it into one continuous dance party that played - and hasn't stopped - on everyone's radio station (which people listened to in the 80s and 90s more than they do now, what with the introduction of technology in which you can listen to your own personal choice of music anytime you want). She combined feminism, religion, sexuality, pop culture, and music like her contemporaries Janet or Cyndi never did: Cyndi was just a girl who wanted to have fun while Janet wanted control. Great messages for little girls, but nothing compared to the impact Madonna had when she was showing girls they could be anything they wanted while having fun and being sexy and not letting anyone tell them what to do, drawing not only them, but their parents and grandparents into the whole discussion. Britney and Gaga and the others who reference Madonna like Madonna referenced Monroe or Dietrich will never be able to achieve the same mythic icon status because of the fragmentation of media that is now the norm in today's world (everyone is watching their own shows and listening to their own music on their own schedules).
If it's a question of talent or sincerity or quality, that is a whole other discussion. But to not realize that Madonna is a pop cultural icon for the ages akin to Presley and Monroe, Brando and the Beatles, or Bowie or Springsteen - a fame in which the name alone evokes fame and carries force just by itself - is just being incredibly ignorant. Janet may be a better singer, for instance, but her influence was not - and never will be - on par with the influence Madonna has had. Monroe was not a great actor - neither was Brando or Dean for that matter - but they, like Madonna, tapped into something quite vital in the mythology of Western history. All you have to do is go in the cultural section of the library and see oodles of books written on Madonna by sociologists and academicians who are trying to find meaning in her videos and theorize ad naseum the social impact of her pop politics. Janet, Cyndi and all the others have never inspired that kind of response. No matter how badly her most rabid haters want it, Madonna is here to stay even way past her mortal expiry date.