2008 and 2011 hands down.
2008 had the 5 strong females - Alexandra, Ruth, Diana, Laura and Rachel - as well as JLS and even Austin was pretty good.
2011 had the best final line-up to date with Little Mix, Marcus and Amelia all being entertaining acts and all rightly made the final. In addition to them, Misha, Janet (not my cup of tea but she was unique for XF), Kitty and The Risk.
This year is also looking to be promising (but it's too early to tell) with Alien, 4th Impact, Seann Miley Moore, Lauren, Louisa, Kiera, Mason and Anton.