Arthur sang U2's hit 'One' before singing Frankie Goes to Hollywood classic 'The Power of Love'.
Following his rendition of the song, which has been more recently sung by Gabrielle Aplin and features on John Lewis's Christmas advert, Barlow said: "James, this is simple - that was the performance of the series.
"Amazing. It really was. You make people emote - you bring out the emotion in people. Well done."
Louis Walsh added that Arthur was "credible" and had "the X factor", while Tulisa said that she felt "so proud" to be a judge on the programme while Arthur was still a contestant.
The 'Sight of You' singer also said she would be "even more proud" to see Arthur crowned the winner of The X Factor next week.
Arthur said that he had "never wanted anything more" than winning the competition.
Watch James Arthur perform 'The Power of Love' on The X Factor below:
Photo gallery - The X Factor semi-final in pictures:
Copyright: Ken McKay/Thames/Rex Features