If it's UK or US No1s.......(these are all UK No1s lol!)
Rachel - Just Like A Pill (Pink) - maybe obvious, but would sound cool and be unique to XF
Daniel - Ruby (Kaiser Chiefs) - Would be unexpected but could suit him and keep him current
Ruth - Stay (Shakespeare's Sister) - Love the Spanish singing but she shouldn't necessarily do it every week, she could even sing this in Spanish well without it being predictable, great underused song. I think she's doing Hips Don't Lie though - it was mentioned on Xtra
JLS - Closer (Ne-Yo) - It kept No Air off No1, and JLS murdered No Air at "Louis's house" - perfect fit

BadLashes - I Kissed A Girl (Katy Perry) - If they want to be the slightly "edgier" girlband lol

Girlband - These Words (Natasha Bedingfield) - Interesting possibilties for harmonies, cool, not overused song
Diana - Punk Rocker (Sandi Thom) - Would suit her voice and personality but be slightly different for her, she's improving

Alexandra - Take A Bow (Rihanna) - Might help her show more personality, and be a change from Whitney.
Laura - Alone (Heart) - Surprisingly underused here, would suit her voice and be a change from some of the songs she's done lately, though maybe something she could "make her own" more would be even better
Austin - Crazy (Gnarls Barkley) - He could sing and perform this well, and make it his own.
Eoghan - All About You (McFly) - Young and fun, could suit his voice.
Scott - Are You Ready For Love? (Elton John) - Just browsing he list, would suit his voice and be different/upbeat without sounding out fo place.
Hopefully I'll be able to come up with better ideas for some of the acts, but I'm happy with those for now lol
xxxx