#4 William Gallas - Ashley Cole
Chelsea gave Arsenal £5 million and William Gallas to bring Ashley Cole to Stamford Bridge. Ashley Cole and William Gallas were a huge success at Arsenal and Chelsea respectively with two Premier League titles each.
The swap was expected to benefit both clubs but Chelsea emerged a clear winner a few years later. William Gallas made 101 Premier League appearances in his four years of stay at Arsenal without much success. He moved to Tottenham Hotspur in 2010.
On the other hand, Ashley Cole made 229 Premier League appearances at Stamford Bridge and went on to win the league, four FA Cups, and the UEFA Champions League with Chelsea. He left Chelsea in 2014 after 8 successful years.
Edited by Raunak J