Series 1 -
Best: Cassie Compton (16-24s)
Worst: G4 (Groups)

Series 2 -
Best: Maria Lawson (Over 25s)
Worst: Chico Slimani (Over 25s)

Series 3 -
Best: Leona Lewis (16-24s)
Worst: The MacDonald Brothers (Groups)

Series 4 -
Best: Niki Evans (Over 25s)
Worst: Same Difference (Groups)

Series 5 -
Best: Laura White (Girls)
Worst: Daniel Evans (Over 25s)

Series 6 -
Best: Stacey Solomon (Girls)
Worst: John & Edward (Groups)

Series 7 -
Best: Cher Lloyd (Girls)
Worst: Wagner (Over 28s)

Series 8 -
Best: Sophie Habibis (Girls)
Worst: Johnny Robinson (Over 25s)

Series 9 -
Best: Jade Ellis (Girls)
Worst: Christopher Maloney (Over 28s)

Series 10 -
Best: Tamera Foster (Girls)
Worst: Luke Friend (Boys)

Series 11 -
Best: Stephanie Nala (Girls)
Worst: Stevi Ritchie (Over 25s)

2004 Rowetta and Two To Go

2005 Shayne and Addictiv Ladies

2006 Ray and Unconventionals

2007 Rhydian and Daniel de Bourg

2008 JLS and Daniel/Eoghan

2009 Joe/Jamie and Miss Frank

2010 John Adeleye and Wagner

2011 Marcus and Johnny/Frankie

2012 Jahmene and Lucy

2013 Luke and Lorna

2014 Andrea and Blonde Electra

2004
Best: Roberta Howett
Worst: Verity Keays

2005
Best: Shayne Ward (Maria comes close)
Worst: Chico

2006
Best: Leona Lewis
Worst: Ray Quinn

2007
Best: Rhydian Roberts
Worst: Leon Jackson

2008
Best: Ruth Lorenzo
Worst: Eoghan Quigg

2009
Best: Olly Murs
Worst: Joe McElderry

2010
Best: Matt Cardle (Rebecca comes close)
Worst: Mary Byrne

2011
Best: Misha B
Worst: Marcus Collins

2012
Best: James Arthur (Ella comes close)
Worst: Chris Maloney/Lucifer

2013
Best: Tamera Foster
Worst: Nicholas McDonald

2014
Best: Lauren Platt
Worst: Stereo Kicks (though I will eternally love Tom Mann <3)

2015
Best: Louisa Johnson
Worst: Anton

2004
Didnt watch

2005
Best: Shayne Ward
Worst: Chico

2006
Best: Leona Lewis
Worst: McDonald Brothers

2007
Can only really remember Leon, Rhydian and Same Difference lol

2008
Best: Diana Vickers/JLS
Worst: Eoghan Quigg/Daniel Evans

2009
Best: Olly Murs/Joe/Stacey
Worst: Rikki Loney/LLoyd.

2010
Best: One Direction/Aiden Grimshaw
Worst: Wagner

2011
Best: Little Mix/Amelia
Worst: Johnny Robinson (hope i have correct year lol)
2012
Best: Ella Henderson/Union J
Worst: Chris Maloney - worst ever.

2013
Best: Luke Friend - I did personally like Sam C aswell (*insert embarresed smiley)
Worst: The Blonde woman that went 2nd - Shelley or something like that.

2014
Best: Ben
Worst: Stereo Kicks

2015
Will see after a few live shoes.

2004
Best - Cassie
Worst = 2 To Go

2005
Best = Andy Abraham
Worst = Philip Magee

2006
Best = Leona Lewis
Worst = The McDonald Brothers

2007
Best = Rhydian
Worst = Leon (Worst. Winner. Ever!!!)

2008
Best = Ruth Lorenzo
Worst = Eoghan Quigg

2009
Best = Joe McElderry
Worst = Rikki Loney

2010
Best = Matt Cardle
Worst = Storm Lee

2011
Best = Little Mix
Worst = Frankie Cocozza

2012
Best = Ella Henderson
Worst = Jahméne Douglas

2013
Best = Nicholas McDonald
Worst = Abi Alton

2014
Best = Fleur East
Worst = Stereo Kicks

2015 (based on what I've seen so far)
Best = Seann Miley Moore
Worst = Bupsi

Series 1
Favourite: Verity Keays
Least Favourite: Tabby Callaghan

Series 2
Favourite: Nicholas Dorsett
Least Favourite: Phillip Magee

Series 3
Favourite: Ashley McKenzie
Least Favourite: Dionne Mitchell

Series 4
Favourite: Niki Evans
Least Favourite: Emily Nakanda

Series 5
Favourite: Daniel Evans
Least Favourite: Eoghan Quigg

Series 6
Favourite: Miss Frank
Least Favourite: John & Edward

Series 7
Favourite: John Adeleye
Least Favourite: Nicolo Festa

Series 8
Favourite: Sophie Habibis
Least Favourite: James Michael

Series 9
Favourite: Carolynne Poole
Least Favourite: Rylan Clark

Series 10
Favourite: Lorna Simpson
Least Favourite: Luke Friend

Series 11
Favourite: Lola Saunders
Least Favourite: Overload Generation

Series 12
Favourite: Reggie n Bollie
Least Favourite: Max Stone

Phillip Magee - worst of any series ever. Though Lucifer Maloney runs him a close second

Fave / least fave

Series 3 (when I started watching: Leona / Phillip or Ray
Series 4: Alisha / Leon
Series 5: Alexandra / Eoghan
Series 6: Stacey / Jedward
Series 7: Rebecca / Wagner
Series 8: Little Mix / Craig
Series 9: Ella / Jahmene and Christopher (equally horrendous)
Series 10: Tamera / Nicholas
Series 11: Fleur / Jay or Ben
Series 12: Louisa / Anton

I'll start from when I properly started watching and was old enough to remember the performances.

Series 4
Favourite - Rhydian Roberts
Least favourite - Andy Williams

Series 5
Favourite - Ruth Lorenzo
Least favourite - Eoghan Quigg

Series 6
Favourite - Jedward
Least favourite - Olly Murs

Series 7
Favourite - Aiden Grimshaw
Least favourite - Katie Waissel

Series 8
Favourite - Amelia Lily
Least favourite - Frankie Cocozza

Series 9
Favourite - Kye Sones
Least favourite - Jahmene Douglas

Series 10
Favourite - Luke Friend
Least favourite - Sam Bailey

Series 11
Favourite - Lauren Platt
Least favourite - Jay James

Series 1
Favourite - Rowetta Satchel
Least favourite - Verity Keays

Series 2
Favourite - Maria Lawson
Least favourite - Phillip Magee

Series 3
Favourite - Leona Lewis
Least favourite - The MacDonald Brothers

Series 4
Favourite - Same Difference
Least favourite - Leon Jackson

Series 5
Favourite - Ruth Lorenzo
Least favourite - Eoghan Quigg

Series 6
Favourite - Stacey Solomon
Least favourite - Lloyd Daniels

Series 7
Favourite - Aiden Grimshaw
Least favourite - Katie Waissel

Series 8
Favourite - Little Mix
Least favourite - Misha B

Series 9
Favourite - Ella Henderson
Least favourite - Christopher Maloney

Series 10
Favourite - Tamera Foster
Least favourite - Nicholas McDonald

Series 11
Favourite - Fleur East
Least favourite - Stereo Kicks

Series 3

Favourite - Leona Lewis
Least Favourite - Macdonald Brothers

Series 4
Favourite - Alisha Bennett
Least Favourite - Rhydian

Series 5

Favourite - Alexandra Burke
Least Favourite - Diana Vickers

Series 6

Favourite - Stacey Solomon
Least Favourite - Joe Mcelderry

Series 7

Favourite - Cher Lloyd
Least Favourite - Katie Waissel

Series 8

Favourite - Little Mix
Least Favourite - Sophie Habbibis

Series 9
Favourite - James Arthur
Least Favourite - Christopher Maloney

Series 10
Favourite - Tamera Foster
Least Favourite - Nicholas McDonald

Series 11
Favourite - Fleur East
Least Favourite - Chloe Jasmine

2006
Favourite: Leona Lewis
Least: I didn't really watch so anyone of this season that's not her.

2007
Favourite: Niki Evans
Least Favourite: Same Difference

2008
Favourite: Diana Vickers
Least Favourite: Girlband/Daniel Evans

2009
Favourite:
Stacey Solomon
Least Favourite: Kandy Rain

2010
Favourite: TreyC Cohen
Least Favourite: Wagner/Storm/John

2011:
Favourite: Little Mix
Least Favourite: Jon Jo Kerr

2012:
Favourite: Melanie Masson
Least Favourite: Chris Maloney and GMD4 or whatever their name was

2013:
Favourite: Sam Bailey or Tamera Foster
Least: Nicholas or anyone else that year I can't remember zzzzzzzzzzzzzzzzzzzz

2014: Favourite: Fleur East
Least Favourite: Stereokicks

2004
Best: Tabby
Worst: Verity

2006
Best: Leona or Ben or Ashley
Worst: Kerry or Nikita

2007
Best: Rhydian
Worst: Leon

2008
Best: JLS or Ruth
Worst: Daniel

2009
Best: Danyl
Worst: Jedward or Lloyd

2010
Best: Matt or One Direction
Worst: Cher or Katie

2011
Best: Sophie or Little Mix
Worst: Frankie, Johnny or Sami

2012
Best: Melanie
Worst: Christopher Maloney

2013
Best: Luke Friend
Worst: Nicholas or Tamera or Sam C

2014
Best: Paul
Worst: Jake / 8 piece boyband

2015
Best: Max
Worst: Keira

Series 1

Favourite: Roberta Howett
Least Favourite: Voices with Soul

Series 2

Favourite: Brenda Edwards
Least Favourite: Philip Magee

Series 3

Favourite: Leona Lewis/Dionne Mithchell
Least Favourite: Ashley McKenzie

Series 4

Favourite: Same Difference/Rhydian Roberts
Least Favourite: Futureproof


Series 5

Favourite: Ruth Lorenzo
Least Favourite: Eoghan Quigg

Series 6

Favourite; Miss Frank
Least Favourite: Lloyd Daniels

Series 7

Favourite: Mary Byrne/Aiden Grimshaw
Least Favourite: One Direction

Series 8

Favourite: Kitty Brucknell
Least Favourite: Frankie Cocozza

Series 9

Favourite: Carolynne Poole by default
Least Favourite: The final 5

Series 10

Favourite: Sam Bailey
Least Favourite: Tamera Foster

Series 11

Favourite: Only the Young
Least Favourite: Stereo Kicks

Series 12

Favourite: 4th Impact
Least Favourite: Mason Noise

Series 1 -
Best: Rowetta Satchell
Worst: Verity Keays

Series 2 -
Best: Brenda Edwards
Worst: 4tune

Series 3 -
Best: Eton Road
Worst: The MacDonald Brothers

Series 4 -
Best: Same Difference
Worst: Andy Williams

Series 5 -
Best: Ruth Lorenzo
Worst: Austin Drage

Series 6 -
Best: Stacey Solomon
Worst: Lloyd Daniels

Series 7 -
Best: Matt Cardle
Worst: Wagner

Series 8 -
Best: Sophie Habibis
Worst: Nu Vibe

Series 9 -
Best: Jade Ellis
Worst: Kye Sonnes

Series 10 -
Best: Luke Friend
Worst: Shelley Smith

Series 11 -
Best: Only the Young
Worst: Stevi Ritchie

Sorry can not recall each act from the series.
All i can say
2004 G4 the best ever act
There will/has been good acts and bad acts of cause there has.
For me no one can ever beat G4
I liked Rhydian
Joe Mceldery
Worse for me are James Arthur, Frankie Cozocca(spelling?) Mason Noise

had to go offline so this years singers whom I like
Seann,Anton and Lousia

2004
Best - Rowetta
Worst = Tabby

2005
Best = Shayne
Worst = Philip

2006
Best = Leona Lewis
Worst = The McDonald Brothers

2007
Best = Hope
Worst = Same Difference

2008
Best = Diana Vickers
Worst = Eoghan Quigg

2009
Best = Olly Murs
Worst = Joe McElderry

2010
Best = Cher Lloyd
Worst = Storm Lee

2011
Best = Little Mix
Worst = Janet Devlin

2012
Best = Ella Henderson
Worst = Christopher Maloney

2013
Best = Takers Foster
Worst = Abi Alton

2014
Best = Lola Saunders
Worst = Blonde Electra

2015
Best = Louisa Johnson
Worst = Anton Stephans