7. Denilson 2006-13
Mooted as a future star when he arrived from Sao Paolo in 2006 after regularly captaining the Brazilian under-17 side, he was expected to become a mainstay of the Gunners midfield once he got brought up to speed with the rigours of English football.
But, he never did quite catch up and he frequently put in mediocre displays that caused Arsene Wenger to lose patience with him, loaning him back to Sao Paolo for a season before selling him back there permanently over the summer.