Surely Bale?
In my lifetime probably one of Sheringham or Anderton - they cost £2m each and both nearly clocked up a decade as our star performers.
I'm not saying Lineker, Klinsmann, Gascoigne, Ginola, Berbatov, Modric and Rafa weren't better players, but they were more expensive and/or stayed for much shorter periods.
Ginola only cost like £2m or something if I recall
In my lifetime, Jimmy Greaves.
But if Bale stays and continues to produce, maybe ...
Ossie and Ricky double signing. Two exotic World Cup winners at a time when foreign signings were unheard of