Kelly Clarkson
Leona Lewis
Any song with a key change in the middle and a choir
Aint no sunshine
Michael Jackson

Add to that:

Anything on next season of Glee. ( Will never forget seeing the ads for Glee and thinking ah so that's why Joe McElderry had such a random song cos Cowell was cashing in on the future success of Glee)

Westlife
Robbie Williams
Christina Aguilera ballads for the girls!

Anything that has a note that lasts for about a minute
You Are So Beauitful by Joe Cocker

He loves 'If Your Not The One' by Daniel Bedingfield, and called it the 'perfect' popsong once, but oddly not many of his talent show acts have attempted it.

The famous Stand by me and Beautiful Girls remix, Used by 2 XF Contestants.

and David Archuleta on American Idol...yet each time its used Simon always appears 'stunned' at such a unique rendition

'Somewhere' from West Side Story

I think he only uses it for People lots of Teenagers fancy (Lloyd Daniels and JLS)