I can't really talk about the first four seasons because I didn't watch them [I only looked at Leona's performances on Youtube].

S5 (2008): Alexandra Burke - "Without You" or "Listen"
S6 (2009): Joe McElderry - "Don't Let The Sun Go Down On Me"
S7 (2010): Matt Cardel - "First Time (Ever I Saw Your Face)"
S8 (2011): Little Mix - "ET" or "Don't Let Go"
S9 (2012): James Arthur - "Let's Get It On"
S10 (2013): Sam Bailey - "Listen" (audition) or "Bleeding Love"
S11 (2014): Ben Haenow - "Man In The Mirror"
S12 (2015): Louisa Johnson - "This Is A Man's Man's World"

S3 (2006): Leona Lewis - Summertime
S4 (2007): Leon Jackson - You Don't Know Me
S5 (2008): Alexandra Burke - Listen
S6 (2009): Joe McElderry - Don't Stop Believing
S7 (2010): Matt Cardel - First Time (Ever I Saw Your Face)
S8 (2011): Little Mix - E.T
S9 (2012): James Arthur - Hometown Glory
S10 (2013): Sam Bailey - That Gloria Gaynor song that Saara sung the other week
S11 (2014): Ben Haenow - Jealous Guy
S12 (2015): Louisa Johnson - This Is A Man's Man's World

They were all alright winners. I think Leon just got the mums voting for him and seemed a nice lad, he could sing but wasn't majorly impressive unfortunately. Ben wasn't great, I think his looks and just being a nice guy won it for him. Sam Bailey was really good, she just struggled to make a career for herself. Overall, the winners aren't too bad. Even the bad ones aren't that bad (Steve Brookstein aside).

S1 Steve Brookstein - Sharon ripping him a new one
S2 Shayne Ward - Cry Me A River
S3 Leona Lewis - Summertime
S4 Leon Jackson - The Kylie duet
S5 Alexandra Burke - Listen with Beyonce
S6 Joe McElderry - I switched off before the final - Didn't get any of the last few acts
S7 Matt Cardel - First Time Ever I Saw Your Face
S8 Little Mix - E.T/Hold On
S9 James Arthur - Hometown Glory/Lets Get It On
S10 Sam Bailey - Didn't get her at all. Didn't like any of her performances
S11 Ben Haenow - Thinking Out Loud
S12 Louisa Johnson - Wasn't a fan. I found her very meh

For Alexandra it was "Candyman" in week 3.
It was at that early point that she shaked off the boring "diva ballad" tag and showed she was an all rounder and contemporary pop star.