(1) Simon Cowell
The most famous talent show judge in the world probably, Simon Cowell has been a permanent fixture on all 7 series to date. Known of course for his frank and honest feedback on the performance of the artists he seems to have mellowed slightly in recent years, no longer needing to have that persona all the time as he is rich enough not to need to stict firmly to the act, and of course either runs the production side of most of the shows or is more than just a judge anyway, and you don't sack yourself.
(2) Louis Walsh
The Irish judge Louis Walsh who has managed many successful bands, mostly Irish of course, over the years has also been a permanent fixture. There were rumours he was going to be dropped before most series but in the end he has appeared on all of them, and apparently often falls out with Simon, although whether that is just for show or not of course only someone who actually knows them could say.
(3) Sharon Osbourne
Was a judge on several series before being replaced by Cheryl Cole. Sat next to Louis, she is said to have fallen out with other judge Dannii Minogue with the two reportedly not particularly liking each other for whatever reason, presumably a clash of egos!
(4) Dannii Minogue
Kylie's less-well known and less successful sister, Dannii Minogue has nevertheless landed the plumb job of being an X-factor judge and is now known more her style apparently than necessarily her ability to offer an accurate and insightful critique of vocal abilities.
(5) Cheryl Cole
She has been a judge for the last few shows and perhaps has had the best category each year either through luck or perhaps intentional action on behalf of the producers. Seemingly a very popular addition to the show and clearly Simon Cowell and her get on well and he enjoys having her on the show; has had some criticism for refusing to cast a vote at the end of the show when two of her acts are in the bottom two which is of course the most interesting part of the show where the judges have to decide who to kick out in the earlier rounds.