If there are big names and great songs, the guest stars on Sunday can be a real highlight of the live shows. What are your favourite of all time?

I'll throw in a few fantastic ones:
https://www.youtube.com/watch?v=sApQJfZurC4

https://www.youtube.com/watch?v=bvsSk_V1iFM

Bruno Mars - Locked Out Of Heaven

Best live guest performer ever and I'm not normally a fan

Alicia Keyes a close second

Hands down Leona Lewis Run. I watch it weekly no one will compare to the vocals and atmosphere that performance created. Simply stunning!!!

I remember the audience and judges applauding mid-song for the notes she hit on that one

Yep, Bruno for me too, he was great.

And Beyoncé - If I Were a Boy.

Yeah I agree

I wasn't expecting it, I was blown away

Xtina - Express

Bruno Mars - Locked out of Heaven

Beyonce - If I were a boy

I think Leona Lewis - Run is the best vocal performance ever on the show tbh

Beyonce- If I were a Boy

Alicia Keys Medley

I also loved Alexandra Burke Bad Boys

And tbh all Rihanna's X Factor performances have been sensational always brings in when she performs on show. Especially Stay and Diamonds

Also loved AB's performance
Bruno Mars - Looked out of Heaven

for me it would have been buble doing "cry me a river"

that performance almost defined stage presence , he had you hooked from the very first note, and i am not really a fan of his

No - one will top this performance for me as love them both
http://youtu.be/cHTalNt_TAw

It also made me laugh they were bowing down to him & he showed our talent as being mediocre at best.

Run without a doubt.

I think my favourite at the time of airing was Gaga's Bad Romance, since I was still going through a huge Gaga phase at the time.

Run for me too ... by a mile.

Rihanna's had a few great ones:

Diamonds:
https://www.youtube.com/watch?v=Nw-NHqEa4FA

Stay/We Found Love:
https://www.youtube.com/watch?v=rSE2SOjDCN0

Only Girl (In The World):
https://www.youtube.com/watch?v=eZo9e1GeVdQ

What's My Name
https://www.youtube.com/watch?v=HnKKDH8gLKw

I thought John Legend gave a great performance last series.

Oh I just remembered Idina Menzel and Michael Bublé performing like they'd had a few glasses of wine beforehand. I think that one was my favourite

Bruno and Gaga were brilliant.

I absolutely loved Fleur last night though! For a first single it was outstanding!

Will Young performing Grace:

https://www.youtube.com/watch?v=Ks1Y_6P66-4

I don't remember that Katy Perry performance. Must have fast-forwarded through it. Nonetheless it's such a blatant rip off of the Daft Punk Around The World video

I'm struggling to think of any but I'm never really a fan of the acts they tend to get on. Having said that Gaga was always good value for sheer showmanship.

Leona Lewis - Run.