I think you know who No 1 is, but the list is compelling. Disclosure: The list is written by Marvel.
http://www.marvel.com/news/comicstories.2399.Take_10:_Greatest_X-Men
Why She Makes The List: The X-Men's bad girl turned good (?), Emma Frost has turned heads since she first made the scene as the nefarious White Queen of the Hellfire Club. Though we've since learned Emma's motivations have never been as simple as pursuit of money or power, she remains unpredictable and you never know when her ice queen exterior may give way to something more sinister. However, as headmistress of the Xavier Institute and Cyclops' lover, Emma has also shown a softer side—one she'd prefer nobody ever see.
http://www.marvel.com/news/comicstories.2399.Take_10:_Greatest_X-Men
Why She Makes The List: The X-Men's bad girl turned good (?), Emma Frost has turned heads since she first made the scene as the nefarious White Queen of the Hellfire Club. Though we've since learned Emma's motivations have never been as simple as pursuit of money or power, she remains unpredictable and you never know when her ice queen exterior may give way to something more sinister. However, as headmistress of the Xavier Institute and Cyclops' lover, Emma has also shown a softer side—one she'd prefer nobody ever see.