if you look at some of the past winners like shayne ward or leon jackson (leon's name being very similar to leona as well), you just don't here about previous x factor winners like you do about leona lewis and also i think leona lewis was the only x factor winner to break america.
so it is probably true that leona lewis has been the most successful x factor winner there has been.
even alexandra burke has not had the same kind of success as leona lewis, will joe mcelderry get the same success as leona did, probably not, but joe mcelderry will probably do okay compared to some acts who just seem to have disappeared after winning the x factor like leon jackson whose fame seemed to be very short lived.
leona lewis officially the most successful x factor winner, leona lewis won x factor 2006:
alexandra burke the second most successful x factor winner?, alexandra burke won x factor 2008, alexandra burke's x factor song was hallelujah:
shayne ward did quite well off of the x factor and has new songs and albums coming out, never did quite as well as leona lewis though, shayne ward won the x factor 2005:
leon jackson probably did the worst out of the other 3 in this x factor poll, with leon jackson seemingly dropped by his record label after only one album, leon jackson's x factor song back in 2007 x factor was when you believe:
