4. Sergio Aguero | Atletico Madrid to Manchester City | £38m
Comfortably the most expensive signing on this list, but Aguero is unquestionably worth it.
He fired Manchester City to their first ever Premier League title, he's hit 49 league goals in less than 100 appearances and he's easily one of the top five strikers in the world.
The only thing stopping him from appearing much higher on this list? Can he do it for the next five to ten years? That and his mysteriously quiet 2012-13 season. Remember that?
There's no reason to doubt it long-term though, but until he's done it, he stays behind the likes of other more established forwards.