(1) Kate Thornton
She presented the original few series of the X-Factor, that is from 2004 - 2006. Her last series was the one that was won by Leona Lewis who has probably been the most successful of all the people who have been on the X-Factor.
(2) Dermot O'Leary
He was brought in to replace Kate Thornton in 2007 and has been there ever since as the presenter. It was definitely a good move to change presenter, as Dermot is much better at presenting the show than Kate and it feels like a much better fit for the show having Dermot hosting it.
(3) Ben Shepard
He hosted this show in the first few series when it was much more of a separate spin off and less watched than it is now, so many people won't be aware that he ever presented the show. Apparently he left after being annoyed he did not get the main hosting job after Kate Thornton was removed from the presenting job, but it was a wise move as Dermot is definitely a more accomplished and authoritative presenter.
(4) Fearne Cotton
She presented just one series of the Xtra Factor, which was Series Four, and again as it was just one series then many people won't be aware that she ever actually presented the show.
(5) Holly Willoughby
Holly presented two series of the show, and is of course a good friend of Fearne. Presumably she had to leave the show due to her success elsewhere in landing the job of presenting This Morning alongside Phillip Schofield after the departure of Fern Britton.
(6) Konnie Huq
The current host at time of writing, for series 7 of the X-Factor, Konnie may be familiar to anyone who has ever watched Blue Peter as she used to be the presenter of that most famous of children's TV shows.